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RFC 2421

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Voice Profile for Internet Mail - version 2

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Obsoleted by:    3801
Obsoletes:    1911

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Network Working Group                                       G. Vaudreuil
Request for Comments: 2421                           Lucent Technologies
Obsoletes: 1911                                               G. Parsons
Category: Standards Track                               Northern Telecom
                                                          September 1998

              Voice Profile for Internet Mail - version 2

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.


   This document profiles Internet mail for voice messaging.  It
   obsoletes RFC 1911 which describes version 1 of the profile.  A list
   of changes from that document are noted in Appendix F.  As well,
   Appendix A summarizes the protocol profiles of this version of VPIM.

   Please send comments on this document to the EMA VPIM Work Group
   mailing list:  <>

Working Group Summary

   This profile is not the product of an IETF working group, though
   several have reviewed the document.  It is instead the product of the
   VPIM Work Group of the Electronic Messaging Association (EMA).  This
   work group, which has representatives from most major voice mail
   vendors and several email vendors, has held several interoperability
   demonstrations between voice messaging vendors and is currently
   promoting VPIM trials and deployment.

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Table of Contents

   1. ABSTRACT .........................................................3
   2. SCOPE ............................................................3
     2.1 Voice Messaging System Limitations ............................3
     2.2 Design Goals ..................................................4
   3. PROTOCOL RESTRICTIONS ............................................5
   4. VOICE MESSAGE INTERCHANGE FORMAT .................................6
     4.1 Message Addressing Formats ....................................6
     4.2 Message Header Fields .........................................9
     4.3 Voice Message Content Types ..................................15
     4.4 Other Message Content Types ..................................21
     4.5 Forwarded Messages ...........................................23
     4.6 Reply Messages ...............................................23
     4.7 Notification Messages ........................................24
   5. MESSAGE TRANSPORT PROTOCOL ......................................24
     5.1 ESMTP Commands ...............................................25
     5.2 ESMTP Keywords ...............................................27
     5.3 ESMTP Parameters - MAIL FROM .................................28
     5.4 ESMTP Parameters - RCPT TO ...................................29
     5.5 ESMTP - SMTP Downgrading .....................................29
   6. DIRECTORY ADDRESS RESOLUTION ....................................30
   7. IMAP ............................................................30
   8. MANAGEMENT PROTOCOLS ............................................30
     8.1 Network Management ...........................................31
   9. CONFORMANCE REQUIREMENTS ........................................31
   10. SECURITY CONSIDERATIONS ........................................32
     10.1 General Directive ...........................................32
     10.2 Threats and Problems ........................................32
     10.3 Security Techniques .........................................33
   11. REFERENCES .....................................................33
   12. ACKNOWLEDGMENTS ................................................36
   13. AUTHORS' ADDRESSES .............................................36
   14. APPENDIX A - VPIM REQUIREMENTS SUMMARY .........................37
   15. APPENDIX B - EXAMPLE VOICE MESSAGES ............................45
   18. APPENDIX E - IANA REGISTRATIONS ................................52
     18.1 vCard EMAIL Type Definition for VPIM ........................52
     18.2 Voice Content-Disposition Parameter Definition ..............52
   20. FULL COPYRIGHT NOTICE ..........................................56

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1. Abstract

   A class of special-purpose computers has evolved to provide voice
   messaging services.  These machines generally interface to a
   telephone switch and provide call answering and voice messaging
   services.  Traditionally, messages sent to a non-local machine are
   transported using analog networking protocols based on DTMF signaling
   and analog voice playback.  As the demand for networking increases,
   there is a need for a standard high-quality digital protocol to
   connect these machines.  The following document is a profile of the
   Internet standard MIME and ESMTP protocols for use as a digital voice
   messaging networking protocol. The profile is referred to as VPIM
   (Voice Profile for Internet Mail) in this document.

   This profile is based on earlier work in the Audio Message
   Interchange Specification (AMIS) group that defined a voice messaging
   protocol based on X.400 technology.  This profile is intended to
   satisfy the user requirements statement from that earlier work with
   the industry standard ESMTP/MIME mail protocol infrastructures
   already used within corporate intranets. This second version of VPIM
   is based on implementation experience and obsoletes RFC 1911 which
   describes version 1 of the profile.

2. Scope

   MIME is the Internet multipurpose, multimedia messaging standard.
   This document explicitly recognizes its capabilities and provides a
   mechanism for the exchange of various messaging technologies,
   primarily voice and facsimile.

   This document specifies a restricted profile of the Internet
   multimedia messaging protocols for use between voice processing
   server platforms.  These platforms have historically been special-
   purpose computers and often do not have the same facilities normally
   associated with a traditional Internet Email-capable computer.  As a
   result, VPIM also specifies additional functionality as it is needed.
   This profile is intended to specify the minimum common set of
   features to allow interworking between compliant systems.

2.1 Voice Messaging System Limitations

   The following are typical limitations of voice messaging platform
   which were considered in creating this baseline profile.

     1) Text messages are not normally received and often cannot be
     easily displayed or viewed.  They can often be processed only via
     text-to-speech or text-to-fax features not currently present in
     many of these machines.

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     2) Voice mail machines usually act as an integrated Message
     Transfer Agent, Message Store and User Agent.  There is no relaying
     of messages, and RFC 822 header fields may have limited use in the
     context of the limited messaging features currently deployed.

     3) Voice mail message stores are generally not capable of
     preserving the full semantics of an Internet message.  As such, use
     of a voice mail machine for gatewaying is not supported.  In
     particular, storage of recipient lists, "Received" lines, and
     "Message-ID" may be limited.

     4) Internet-style distribution/exploder mailing lists are not
     typically supported.  Voice mail machines often implement only
     local alias lists, with error-to-sender and reply-to-sender
     behavior.  Reply-all capabilities using a CC list are not generally

     5) Error reports must be machine-parsable so that helpful responses
     can be voiced to users whose only access mechanism is a telephone.

     6) The voice mail systems generally limit address entry to 16 or
     fewer numeric characters, and normally do not support alphanumeric
     mailbox names.  Alpha characters are not generally used for mailbox
     identification as they cannot be easily entered from a telephone

2.2 Design Goals

   It is a goal of this profile to make as few restrictions and
   additions to the existing Internet mail protocols as possible while
   satisfying the requirements for interoperability with current
   generation voice messaging systems.  This goal is motivated by the
   desire to increase the accessibility to digital messaging by enabling
   the use of proven existing networking software for rapid development.

   This specification is intended for use on a TCP/IP network; however,
   it is possible to use the SMTP protocol suite over other transport
   protocols.  The necessary protocol parameters for such use is outside
   the scope of this document.

   This profile is intended to be robust enough to be used in an
   environment, such as the global Internet with installed-base gateways
   which do not understand MIME, though typical use is expected to be
   within corporate intranets.  Full functionality, such as reliable
   error messages and binary transport, will require careful selection
   of gateways (e.g., via MX records) to be used as VPIM forwarding
   agents.  Nothing in this document precludes use of general purpose
   MIME email packages to read and compose VPIM messages.  While no

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   special configuration is required to receive VPIM compliant messages,
   some may be required to originate compliant structures.

   It is expected that a VPIM messaging system will be managed by a
   system administrator who can perform TCP/IP network configuration.
   When using facsimile or multiple voice encodings, it is suggested
   that the system administrator maintain a list of the capabilities of
   the networked mail machines to reduce the sending of undeliverable
   messages due to lack of feature support.  Configuration,
   implementation and management of these directory listing capabilities
   are local matters.

3. Protocol Restrictions

   This protocol does not limit the number of recipients per message.
   Where possible, server implementations should not restrict the number
   of recipients in a single message.  It is recognized that no
   implementation supports unlimited recipients, and that the number of
   supported recipients may be quite low.

   This protocol does not limit the maximum message length.
   Implementers should understand that some machines will be unable to
   accept excessively long messages.  A mechanism is defined in the RFC
   1425 SMTP service extensions to declare the maximum message size

   The message size indicated in the ESMTP SIZE parameter is in bytes,
   not minutes or seconds.  The number of bytes varies by voice encoding
   format and includes the MIME wrapper overhead.  If the length must be
   known before sending, an approximate translation into minutes or
   seconds can be performed if the voice encoding is known.

   The following sections describe the restrictions and additions to
   Internet mail protocols that are required to be compliant with this
   VPIM v2 profile. Though various SMTP, ESMTP and MIME features are
   described here, the implementer is referred to the relevant RFCs for
   complete details. It is also advisable to check for IETF drafts of
   various Internet Mail specifications that are later than the most
   recent RFCs since, for example, MIME has yet to be published as a
   full IETF Standard. The table in Appendix A summarizes the protocol
   details of this profile.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [REQ].

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4. Voice Message Interchange Format

   The voice message interchange format is a profile of the Internet
   Mail Protocol Suite.  Any Internet Mail message containing the format
   defined in this section is referred to as a VPIM Message in this
   document.  As a result, this document assumes an understanding of the
   Internet Mail specifications.  Specifically, VPIM references
   components from the message format standard for Internet messages
   [RFC822], the Multipurpose Internet Message Extensions [MIME], the
   X.400 gateway specification [X.400], delivery status and message
   disposition notifications [REPORT][DSN][DRPT][STATUS][MDN], and the
   electronic business card [MIMEDIR][VCARD].

4.1 Message Addressing Formats

   RFC 822 addresses are based on the domain name system.  This naming
   system has two components: the local part, used for username or
   mailbox identification; and the host part, used for global machine

4.1.1 VPIM Addresses

   The local part of the address shall be a US-ASCII string uniquely
   identifying a mailbox on a destination system.  For voice messaging,
   the local part is a printable string containing the mailbox ID of the
   originator or recipient.  While alpha characters and long mailbox
   identifiers are permitted, most voice mail networks rely on numeric
   mailbox identifiers to retain compatibility with the limited 10 digit
   telephone keypad.  As a result, some voice messaging systems may only
   be able to handle a numeric local part.  The reception of
   alphanumeric local parts on these systems may result in the address
   being mapped to some locally unique (but confusing to the recipient)
   number or, in the worst case the address could be deleted making the
   message un-replyable.  Additionally, it may be difficult to create
   messages on these systems with an alphanumeric local part without
   complex key sequences or some form of directory lookup (see 6).

   The use of the domain naming system should be transparent to the
   user.  It is the responsibility of the voice mail machine to lookup
   the fully-qualified domain name (FQDN) based on the address entered
   by the user (see 6).

   In the absence of a global directory, specification of the local part
   is expected to conform to international or private telephone
   numbering plans.  It is likely that private numbering plans will
   prevail and these are left for local definition.  However, it is
   RECOMMENDED that public telephone numbers be noted according to the
   international numbering plan described in [E.164]. The indication

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   that the local part is a public telephone number is given by a
   preceding `+' (the `+' would not be entered from a telephone keypad,
   it is added by the system as a flag).  Since the primary information
   in the numeric scheme is contained by the digits, other character
   separators (e.g.  `-') may be ignored (i.e. to allow parsing of the
   numeric local mailbox) or may be used to recognize distinct portions
   of the telephone number (e.g. country code).  The specification of
   the local part of a VPIM address can be split into the four groups
   described below:

     1) mailbox number
        - for use as a private numbering plan (any number of digits)
        - e.g.

     2) mailbox number+extension
        - for use as a private numbering plan with extensions
          any number of digits, use of `+' as separator
        - e.g.

     3) +international number
        - for international telephone numbers conforming to E.164
          maximum of 15 digits
        - e.g.

     4) - for international telephone numbers conforming to E.164
          maximum of 15 digits, with an extension (e.g. behind a
          PBX) that has a maximum of 15 digits.
        - e.g.

   Note that this address format is designed to be compatible with
   current usage within the voice messaging industry.  It is not
   compatible with the addressing formats of RFCs 2303-2304.  It is
   expected that as telephony services become more widespread on the
   Internet, these addressing formats will converge.

4.1.2 Special Addresses

   Special addresses are provided for compatibility with the conventions
   of Internet mail.  These addresses do not use numeric local
   addresses, both to conform to current Internet practice and to avoid
   conflict with existing numeric addressing plans. Two special
   addresses are RESERVED for use as follows:


   By convention, a special mailbox named "postmaster" MUST exist on all
   systems.  This address is used for diagnostics and should be checked
   regularly by the system manager. This mailbox is particularly likely

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   to receive text messages, which is not normal on a voice processing
   platform.  The specific handling of these messages is an individual
   implementation choice.


   If a reply to a message is not possible, such as a telephone
   answering message, then the special address "non-mail-user" must be
   used as the originator's address.  Any text name such as "Telephone
   Answering", or the telephone number if it is available, is permitted.
   This special address is used as a token to indicate an unreachable
   originator. For compatibility with the installed base of mail user
   agents, implementations that generate this special address MUST send
   a negative delivery status notification (DSN) for reply messages sent
   to the undeliverable address.  The status code for such NDN's is
   5.1.1 "Mailbox does not exist".


       From: Telephone Answering <>

4.1.3 Distribution Lists

   There are many ways to handle distribution list (DL) expansions and
   none are 'standard'.  Simple alias is a behavior closest to what most
   voice mail systems do today and what is to be used with VPIM
   messages.  That is:

     Reply to the originator - (Address in the RFC822 Reply-to or From
     Errors to the submitter - (Address in the MAIL FROM: field of the
                                ESMTP exchange and the Return-Path:
                                RFC 822 field)

   Some proprietary voice messaging protocols include only the recipient
   of the particular copy in the envelope and include no "header fields"
   except date and per-message features.  Most voice messaging systems
   do not provide for "Header Information" in their messaging queues and
   only include delivery information.  As a result, recipient
   information MAY be in either the To or CC header fields. If all
   recipients cannot be presented (e.g. unknown DL expansion) then the
   recipient header fields MUST be omitted to indicate that an accurate
   list of recipients (e.g. for use with a reply-all capability) is not

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4.2 Message Header Fields

   Internet messages contain a header information block.  This header
   block contains information required to identify the sender, the list
   of recipients, the message send time, and other information intended
   for user presentation.  Except for specialized gateway and mailing
   list cases, header fields do not indicate delivery options for the
   transport of messages.

   Distribution list processors are noted for modifying or adding to the
   header fields of messages that pass through them.  VPIM systems MUST
   be able to accept and ignore header fields that are not defined here.

   The following header lines are permitted for use with VPIM voice

4.2.1 From

   The originator's fully-qualified domain address (a mailbox address
   followed by the fully-qualified domain name).  The user listed in
   this field should be presented in the voice message envelope as the
   originator of the message.

   Systems compliant with this profile SHOULD provide the text personal
   name of the voice message originator in a quoted phrase, if the name
   is available.  Text names of corporate or positional mailboxes MAY be
   provided as a simple string. From [RFC822]


       From: "Joe S. User" <>

       From: Technical Support <>

   The From address SHOULD be used for replies (see 4.6).  However, if
   the From address contains <non-mail-user@domain>, the user SHOULD NOT
   be offered the option to reply, nor should notifications be sent to
   this address.

   Voice mail machines may not be able to support separate attributes
   for the FROM, REPLY-TO, and SENDER header field and the SMTP MAIL
   FROM command, VPIM conforming systems SHOULD set these values to the
   same address.  Use of addresses different than those present in the
   From header field address may result in unanticipated behavior.

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4.2.2 To

   The To header contains the recipient's fully-qualified domain
   address.  There may be one or more To: fields in any message.



   Systems compliant to this profile SHOULD provide a list of recipients
   only if all recipients are provided.  The To header MUST NOT be
   included in the message if the sending message transport agent (MTA)
   cannot resolve all the addresses in it, e.g. if an address is a DL
   alias for which the expansion is unknown (see 4.1.3).  If present,
   the addresses in the To header MAY be used for a reply message to all

   Systems compliant to this profile MAY also discard the To addresses
   of incoming messages because of the inability to store the
   information.  This would, of course, make a reply-to-all capability

4.2.3 Cc

   The cc header contains additional recipients' fully-qualified domain
   addresses. Many voice mail systems maintain only sufficient envelope
   information for message delivery and are not capable of storing or
   providing a complete list of recipients.

   Systems compliant to this profile SHOULD provide a list of recipients
   only if all disclosed recipients can be provided.  The list of
   disclosed recipients does not include those sent via a blind copy. If
   not, systems SHOULD omit the To and Cc header fields to indicate that
   the full list of recipients is unknown.



   Systems compliant to this profile MAY discard the Cc addresses of
   incoming messages as necessary.    If a list of Cc or to addresses is
   present, these addresses MAY be used for a reply message to all

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4.2.4 Date

   The Date header contains the date, time, and time zone in which the
   message was sent by the originator.  The time zone SHOULD be
   represented in a four-digit time zone offset, such as -0500 for North
   American Eastern Standard Time.  This may be supplemented by a time
   zone name in parentheses, e.g., "-0900 (PDT)".  Compliant
   implementations SHOULD be able to convert RFC 822 date and time
   stamps into local time.


       Date: Wed, 28 Jul 96 10:08:49 -0800 (PST)

   The sending system MUST report the time the message was sent. If the
   VPIM sender is relaying a message from a system which does not
   provide a time stamp, the time of arrival at the VPIM system SHOULD
   be used as the date.  From [RFC822]

4.2.5 Sender

   The Sender header field contains the actual address of the originator
   if the message is sent by an agent on behalf of the author indicated
   in the From: field. This header field MAY be sent by VPIM conforming
   system.  If it is present in a VPIM message, the receiving VPIM
   implementation may ignore the field and only present the From header

4.2.6 Return Path

   The Return-path header is added by the final delivering SMTP server.
   If present, it contains the address from the MAIL FROM parameter of
   the ESMTP exchange (see 5.1.2). Any error messages resulting from the
   delivery failure MUST be sent to this address (see [DRPT] for
   additional details).  Note that if the Return-path is null ("<>"),
   e.g. no path, loop prevention or confidential, a notification MUST
   NOT be sent.  If the Return path address is not available (either
   from this header or the MAIL FROM parameter) the From address may be
   used to deliver notifications.

4.2.7 Message-id

   The Message-id header contains a unique per-message identifier.  A
   unique message-id MUST be generated for each message sent from a
   compliant implementation.

   The message-id is not required to be stored on the receiving system.
   This identifier MAY be used for tracking, auditing, and returning

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   receipt notification reports.  From [RFC822]


       Message-id: <>

4.2.8 Reply-To

   If present, the reply-to header provides a preferred address to which
   reply messages should be sent (see 4.6).  Typically, voice mail
   systems can only support one originator of a message so it is
   unlikely that this field can be supported.  A compliant system SHOULD
   NOT send a Reply-To header. However, if a reply-to header is present,
   a reply-to sender message MAY be sent to the address specified (that
   is, overwriting From). From [RFC822] This preferred address of the
   originator must also be provided in the originator's vCard EMAIL
   attribute, if present (see 4.3.3).

4.2.9 Received

   The Received header contains trace information added to the beginning
   of a RFC 822 message by MTAs.  This is the only header permitted to
   be added by an MTA.  Information in this header is useful for
   debugging when using an US-ASCII message reader or a header parsing

   A compliant system MUST add Received header fields when acting as a
   gateway and MUST NOT remove any Received fields when relaying
   messages to other  MTAs or gateways..  These header fields MAY be
   ignored or deleted when the message is received at the final
   destination. From [RFC822]

4.2.10 MIME Version

   The MIME-Version header indicates that the message conforms to the
   MIME message format specification. Systems compliant with this
   specification SHOULD include a comment with the words "(Voice 2.0)".
   RFC 1911 defines an earlier version of this profile and uses the
   token (Voice 1.0). From [MIME1][VPIM1]


       MIME-Version: 1.0 (Voice 2.0)

   This identifier is intended for information only and SHOULD NOT be
   used to semantically identify the message as being a VPIM message.
   Instead, the presence of the content defined in [V-MSG] SHOULD be
   used if identification is necessary.

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4.2.11 Content-Type

   The content-type header declares the type of content enclosed in the
   message. The typical top level content in a VPIM Message SHOULD be
   multipart/voice-message, a mechanism for bundling several components
   into a single identifiable voice message.  The allowable contents are
   detailed in section 4.3 of this document.  From [MIME2]

4.2.12 Content-Transfer-Encoding

   Because Internet mail was initially specified to carry only 7-bit
   US-ASCII text, it may be necessary to encode voice and fax data into
   a representation suitable for that environment.  The content-
   transfer-encoding header describes this transformation if it is
   needed.  Compliant implementations MUST recognize and decode the
   standard encodings, "Binary", "7bit, "8bit", "Base64" and "Quoted-
   Printable".  The allowable content-transfer-encodings are specified
   in section 4.3.  From [MIME1]

4.2.13 Sensitivity

   The sensitivity header, if present, indicates the requested privacy
   level.  The case-insensitive values "Personal" and "Private" are
   specified. If no privacy is requested, this field is omitted.

   If a sensitivity header is present in the message, a compliant system
   MUST prohibit the recipient from forwarding this message to any other
   user.  A compliant system, however, SHOULD allow the responder to
   reply to a sensitive message, but SHOULD NOT include the original
   message content.  The sensitivity of the reply message MAY be set by
   the responder.

   If the receiving system does not support privacy and the sensitivity
   is one of "Personal" or "Private", a negative delivery status
   notification must sent to the originator with the appropriate status
   code indicating that privacy could not be assured. The message
   contents SHOULD  be returned to the sender to allow for a voice
   context with the notification. A non-delivery notification to a
   private message SHOULD NOT be tagged private since it will be sent to
   the originator.  From: [X.400]

4.2.14 Importance

   Indicates the requested importance to be given by the receiving
   system.  The case-insensitive values "low", "normal" and "high" are
   specified.  If no special importance is requested, this header may be
   omitted and the value assumed to be "normal".

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   Compliant implementations MAY use this header to indicate the
   importance of a message and may order messages in a recipient's
   mailbox. From: [X.400]

4.2.15 Subject

   The subject field is often provided by email systems but is not
   widely supported on Voice Mail platforms. For compatibility with text
   based mailbox interfaces, a text subject field SHOULD be generated by
   a compliant implementation but MAY be discarded if present by a
   receiving system.  From [RFC822]

   It is recommended that voice messaging systems that do not support
   any text user interfaces (e.g. access only by a telephone) insert a
   generic subject header of "VPIM Message" for the benefit of text
   enabled recipients.

4.2.16 Disposition-Notification-To

   This header MAY be present to indicate that the sender is requesting
   a receipt notification from the receiving user agent.  This message
   disposition notification (MDN) is typically sent by the user agent
   after the user has listened to the message and consented to an MDN
   being sent



   The presence of a Disposition-notification-to header in a message is
   merely a request for an MDN described in 4.4.5.  The recipients' user
   agents are always free to silently ignore such a request so this
   header does not burden any system that does not support it.  From

4.2.17 Disposition-Notification-Options

   This header MAY be present to define future extensions parameters for
   an MDN requested by the presence of the header in the previous
   section.  Currently no parameters are defined by this document or by
   [MDN].  However, this header MUST be parsed if present, if MDNs are
   supported.  If it contains a extension parameter that is required for
   proper MDN generation (noted with "=required"), then an MDN MUST NOT
   be sent if the parameter is not understood.  See [MDN] for complete

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4.3 Voice Message Content Types

   MIME, introduced in [MIME1], is a general-purpose message body format
   that is extensible to carry a wide range of body parts.  It provides
   for encoding binary data so that it can be transported over the 7-bit
   text-oriented SMTP protocol.  This transport encoding (denoted by the
   Content-Transfer-Encoding header field) is in addition to the audio
   encoding required to generate a binary object.

   MIME defines two transport encoding mechanisms to transform binary
   data into a 7 bit representation, one designed for text-like data
   ("Quoted-Printable"), and one for arbitrary binary data ("Base64").
   While Base64 is dramatically more efficient for audio data, either
   will work.  Where binary transport is available, no transport
   encoding is needed, and the data can be labeled as "Binary".

   An implementation in compliance with this profile SHOULD send audio
   and/or facsimile data in binary form when binary message transport is
   available.  When binary transport is not available, implementations
   MUST encode the audio and/or facsimile data as Base64.  The detection
   and decoding of "Quoted-Printable", "7bit", and "8bit" MUST be
   supported in order to meet MIME requirements and to preserve
   interoperability with the fullest range of possible devices.
   However, if a content is received in a transfer encoding that cannot
   be rendered to the user, an appropriate negative delivery status
   notification MUST be sent.

   The content types described in this section are identified for use
   within the multipart/voice-message content.  This content, which is
   the fundamental part of a VPIM message, is referred to as a VPIM
   voice message in this document.

   Only the contents profiled subsequently can be sent within a VPIM
   voice message construct (i.e., the mulitpart/voice-message content
   type) to form a simple or a more complex structure (several examples
   are given in Appendix B).  The presence of other contents within a
   VPIM voice message is an error condition and SHOULD result in a
   negative delivery status notification.  When multiple contents are
   present within the multipart/voice-message, they SHOULD be presented
   to the user in the order that they appear in the message.

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4.3.1 Multipart/Voice-Message

   This MIME multipart structure provides a mechanism for packaging a
   voice message into one container that is tagged as VPIM v2 compliant.
   The semantic of multipart/Voice-Message (defined in [V-MSG]) is
   identical to multipart/mixed and may be interpreted as that by
   systems that do not recognize this content-type.

   The Multipart/Voice-Message content-type MUST only contain the
   profiled media and content types specified in this section (i.e.
   audio/*, image/*, message/rfc822 and text/directory).  The most
   common will be: spoken name, spoken subject, the message itself,
   attached fax and directory info.  Forwarded messages are created by
   simply using the message/rfc822 construct.

   Conformant implementations MUST send the multipart/voice-message in a
   VPIM message.  In most cases, this Multipart/Voice-Message content
   will be the top level (i.e. in the Content-Type header).  Conformant
   implementations MUST recognize the Multipart/Voice-Message content
   (whether it is a top level content or below a multipart/mixed) and be
   able to separate the contents (e.g. spoken name or spoken subject).

4.3.2 Message/RFC822

   MIME requires support of the Message/RFC822 message encapsulation
   body part.  This body part is used within a multipart/voice-message
   to forward complete messages (see 4.5) or to reply with original
   content (see 4.6). From [MIME2]

4.3.3 Text/Directory

   This content allows for the inclusion of a Versit vCard [VCARD]
   electronic business card within a VPIM message.  The format is
   suitable as an interchange format between applications or systems,
   and is defined independent of the method used to transport it.  It
   provides a useful mechanism to transport information about the
   originator that can be used by the receiving VPIM system (see 6) or
   other local applications

   Each vCard MUST be contained within a Text/Directory content type
   [MIMEDIR] within a VPIM message.  [MIMEDIR] requires that the
   character set MUST be defined as a parameter value (typically us-
   ascii for VPIM) and that the profile SHOULD be defined (the value
   MUST be vCard within VPIM messages).

   Each VPIM message SHOULD be created with a Text/Directory (vCard
   profile) content type that MUST contain the preferred email address,
   telephone number, and text name of the message originator as well as

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   the vCard version.  The vCard SHOULD contain the spoken name and role
   of the originator, as well as the revision date.  Any other vCard
   attribute MAY also be present.  The intent is that the vCard be used
   as the source of information to contact the originator (e.g., reply,
   call).If the text/directory content-type is included in a VPIM
   message, the vCard profile [VCARD] MUST be used and MUST specify at
   least the following attributes:

     TEL  -    Public switched telephone number in international (E.164)
               format (various types, typically VOICE)

     EMAIL -   email address (various types, typically INTERNET; the
               type VPIM is optionally used to denote an address that
               supports VPIM messages(see 18.1))

     VERSION - Indicates the version of the vCard profile.  Version 3.0
               [VCARD] MUST be used.

   The following attributes SHOULD be specified:

     N   -   Family Name, Given Name, Additional Names, Honorific
             Prefixes, and Suffixes. Because it is expected that
             recipients using a telephone user interface will use the
             information in the vCard to identify the originator, and
             the GUI will see the information presented in the FROM
             line, all present components in the text name of the FROM
             header field MUST match the values provided by the Vcard.

     ROLE -  The role of the person identified in `N' or `FN', but may
             also be used to distinguish when the sender is a corporate
             or positional mailbox

     SOUND - spoken name sound data (various types, typically 32KADPCM)

     REV  -  Revision of vCard in ISO 8601 date format

   The vCard MAY use other attributes as defined in [VCARD] or
   extensions attributes not yet defined (e.g. capabilities).

   If present, the spoken name attribute MUST be denoted by a content ID
   pointing to an audio/* content elsewhere in the VPIM message.

   A typical VPIM message (i.e. no forwarded parts), MUST only contain
   one vCard -- more than one is an error condition.  A VPIM message
   that contains forwarded messages, though, may contain multiple
   vCards.  However, these vCards MUST be associated with the
   originator(s) of the forwarded message(s) and the originator of the
   forwarding message.  As a result, all forwarded vCards will be

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   contained in message/rfc822 contents -- only the vCard of forwarding
   originator will be at the top-level.


     Content-Type: text/directory; charset=us-ascii; profile=vCard
     Content-Transfer-Encoding: 7bit

     ORG:Northern Telecom
     VERSION: 3.0

4.3.4 Audio/32KADPCM

   An implementation compliant to this profile MUST send Audio/32KADPCM
   by default for voice [ADPCM].  Receivers MUST be able to accept and
   decode Audio/32KADPCM.  Typically this body contains several minutes
   of message content, however if used for spoken name or subject the
   content should be considerably shorter (i.e. about 10 and 20 seconds

   If an implementation can only handle one voice body, then multiple
   voice bodies (if present) SHOULD be concatenated, and SHOULD NOT be
   discarded.  It is RECOMMENDED that this be done in the same order as
   they were sent. Note that if an Originator Spoken Name audio body and
   a vCard are both present in a VPIM message, the vCard SOUND attribute
   MUST point to this audio body (see 4.3.3).

   While any valid MIME body header MAY be used, several header fields
   have the following semantics when included with this body part: Content-Description:

   This field MAY be present to facilitate the text identification of
   these body parts in simple email readers.  Any values may be used,
   though it may be useful to use values similar to those for Content-


       Content-Description: Big Telco Voice Message

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   This field MUST be present to allow the parsable identification of
   these body parts.  This is especially useful if, as is typical, more
   than one Audio/32KADPCM body occurs within a single level (e.g.
   multipart/voice-message).  Since a VPIM voice message is intended to
   be automatically played upon display of the message, in the order in
   which the audio contents occur, the audio contents must always be of
   type inline.  However, it is still useful to include a filename
   value, so this should be present if this information is available.
   From [DISP]

   In order to distinguish between the various types of audio contents
   in a VPIM voice message a new disposition parameter "voice" is
   defined with the parameter values below to be used as appropriate
   (see 18.2):

     Voice-Message - the primary voice message,
     Voice-Message-Notification - a spoken delivery notification
       or spoken disposition notification,
     Originator-Spoken-Name - the spoken name of the originator,
     Recipient-Spoken-Name - the spoken name of the recipient if
       available to the originator and present if there is ONLY one
     Spoken-Subject- the spoken subject of the message, typically
       spoken by the originator

   Note that there SHOULD only be one instance of each of these types of
   audio contents per message level.  Additional instances of a given
   type (i.e., parameter value) may occur within an attached forwarded
   voice message.

   Implementations that do not understand the "voice" parameter (or the
   Content-Disposition header) can safely ignore it, and will present
   the audio bodyparts in order (but will not be able to distinguish
   between them).


       Content-Disposition: inline; voice=spoken-subject;
                           filename="msg001.726" Content-Duration:

   This field MAY be present to allow the specification of the length of
   the audio bodypart in seconds.  The use of this field on reception is
   a local implementation issue.  From [DUR]

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       Content-Duration: 33 Content-Language:

   This field MAY be present to allow the specification of the spoken
   language of the audio bodypart.  The encoding is defined in [LANG].
   The use of this field on reception is a local implementation issue.

   Example for UK English:

       Content-Language: en-UK

4.3.5 Image/Tiff

   A common image encoding for facsimile, known as TIFF-F, is a
   derivative of the Tag Image File Format (TIFF) and is described in
   several documents.  For the purposes of VPIM, the F Profile of TIFF
   for Facsimile (TIFF-F) is defined in [TIFF-F] and the image/tiff MIME
   content type is defined in [TIFFREG].  While there are several
   formats of TIFF, only TIFF-F is profiled for use in a VPIM voice
   message.  Further, since the TIFF-F file format is used in a store-
   and-forward mode with VPIM, the image MUST be encoded so that there
   is only one image strip per facsimile page.

   All VPIM implementations that support facsimile SHOULD generate
   TIFF-F compatible facsimile contents in the image/tiff;
   application=faxbw sub-type encoding by default.  An implementation
   MAY send this fax content in VPIM voice messages and MUST be able to
   recognize and display it in received messages.  If a fax message is
   received that cannot be rendered to the user (e.g. the receiving VPIM
   system does not support fax), then the system MUST return the message
   with a negative delivery status notification with a media not
   supported status code.

   While any valid MIME body header MAY be used (e.g., Content-
   Disposition to indicate the filename), none are specified to have
   special semantics for VPIM and MAY be ignored.  Note that the content
   type parameter application=faxbw MUST be included in outbound
   messages.  However, inbound messages with or without this parameter
   MUST be rendered to the user (if the rendering software encounters an
   error in the file format, some form of negative delivery status
   notification MUST be sent to the originator).

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4.3.6 Proprietary Voice or Fax Formats

   Proprietary voice or fax encoding formats or other standard formats
   MAY be supported under this profile provided a unique identifier is
   registered with the IANA prior to use (see [MIME4]).  The voice
   encodings should be registered as sub-types of Audio and the fax
   encodings should be registered as sub-types of Image

   Use of any other encoding except audio/32kadpcm or image/tiff;
   application=faxbw reduces interoperability in the absence of explicit
   manual system configuration.  A compliant implementation MAY use any
   other encoding with explicit per-destination configuration.

4.4 Other Message Content Types

   An implementation compliant with this profile MAY send additional
   contents in a VPIM message, but ONLY outside of the multipart/voice-
   message.  The content types described in this section are identified
   for use with this profile. Additional contents not defined in this
   profile MUST NOT be used without prior explicit per-destination
   configuration. If an implementation receives a VPIM message that
   contains content types not specified in this profile, their handling
   is a local implementation issue (e.g. the unknown contents MAY be
   discarded if they cannot be presented to the recipient).  Conversely,
   if an implementation receives a non-VPIM message (i.e., without a
   mulitpart/voice-message content type) with any of the contents
   defined in 4.3 & 4.4, it SHOULD deliver those contents, but the full
   message handling is a local issue (e.g. the unknown contents _or_ the
   entire message MAY be discarded).  Implementations MUST issue
   negative delivery status notifications to the originator when any
   form of non-delivery to the recipient occurs.

   The multipart contents defined below MAY be sent as the top level of
   a VPIM message (with other noted contents below them as required.) As
   well, the multipart/mixed content SHOULD be used as the top level of
   a VPIM message to form a more complex structure (e.g., with
   additional content types).  When multiple contents are present, they
   SHOULD be presented to the user in the order that they appear in the
   message.  Several examples are given in Appendix B.

4.4.1 Multipart/Mixed

   MIME provides the facilities for enclosing several body parts in a
   single message. Multipart/Mixed SHOULD only be used for sending
   complex voice or multimedia messages.  That is, as the top level
   Content-Type when sending one of the following contents (in addition
   to the VPIM voice message) in a VPIM message.  Compliant systems MUST
   accept multipart/mixed body parts.  From [MIME2]

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4.4.2 Text/Plain

   MIME requires support of the basic Text/Plain content type.  This
   content type has limited applicability within the voice messaging
   environment.  However, because VPIM is a MIME profile, MIME
   requirements should be met.  Compliant VPIM implementations SHOULD
   NOT send the Text/Plain content-type.  Compliant implementations MUST
   accept Text/Plain messages, however, specific handling is left as an
   implementation decision. From [MIME2]

   There are several mechanisms that can be used to support text (once
   accepted) on voice messaging systems including text-to-speech and
   text-to-fax conversions.  If no rendering of the text is possible
   (i.e., it is not possible for the recipient to determine if the text
   is a critical part of the message), the entire message MUST be
   returned to the sender with a negative delivery status notification
   and a media-unsupported status code.

4.4.3 Multipart/Report

   The Multipart/Report is used for enclosing human-readable and machine
   parsable notification (e.g. Message/delivery-status) body parts and
   any returned message content. The multipart/report content-type is
   used to deliver both delivery status reports indicating transport
   success or failure and message disposition notifications to indicate
   post-delivery events such as receipt notification. Compliant
   implementations MUST use the Multipart/Report construct. Compliant
   implementations MUST recognize and decode the Multipart/Report
   content type and its components in order to present the report to the
   user.  From [REPORT]

   Multipart/Report messages from VPIM implementations SHOULD include
   the human-readable description of the error as a spoken audio/*
   content (this speech SHOULD also be made available to the
   notification recipient).  As well, VPIM implementations MUST be able
   to handle (and MAY generate) Multipart/Report messages that encode
   the human-readable description of the error as text.  Note that per
   [DSN] the human-readable part MUST always be present.

4.4.4 Message/Delivery-status

   This MIME body part is used for sending machine-parsable delivery
   status notifications.  Compliant implementations MUST use the
   Message/delivery-status construct when returning messages or sending
   warnings.  Compliant implementations MUST recognize and decode the
   Message/delivery-status content type and present the reason for
   failure to the sender of the message.  From [DSN]

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4.4.5 Message/Disposition-notification

   This MIME body part is used for sending machine-parsable receipt
   notification message disposition notifications.  Conforming
   implementations SHOULD use the Message/Disposition-notification
   construct when sending post-delivery message status notifications.
   These MDNs, however, MUST only be sent in response to the presence of
   the Disposition-notification-to header in 4.2.16.  Conforming
   implementations should recognize and decode the Message/Disposition-
   notification content type and present the notification to the user.
   From [MDN]

4.5 Forwarded Messages

   VPIM version 2 explicitly supports the forwarding of voice and fax
   content with voice or fax annotation.  However, only the two
   constructs described below are acceptable in a VPIM message.  Since
   only the first (i.e. message/rfc822) can be recognized as a forwarded
   message (or even multiple forwarded messages), it is RECOMMENDED that
   this construct be used whenever possible.

   Forwarded VPIM messages SHOULD be sent as a multipart/voice-message
   with the entire original message enclosed in a message/rfc822 content
   type and the annotation as a separate Audio/* or image/* body part.
   If the RFC822 header fields are not available for the forwarded
   content, simulated header fields with available information SHOULD be
   constructed to indicate the original sending timestamp, and the
   original sender as indicated in the "From" line.  However, note that
   at least one of "From", "Subject", or "Date" MUST be present.  As
   well, the message/rfc822 content MUST include at least the "MIME-
   Version", and "Content-Type" header fields. From [MIME2]

   In the event that forwarding information is lost through
   concatenation of the original message and the forwarding annotation,
   such as must be done in a gateway between VPIM and the AMIS voice
   messaging protocol, the entire audio content MAY be sent as a single
   Audio/* segment without including any forwarding semantics.

4.6 Reply Messages

   Replies to VPIM messages (and Internet mail messages) are addressed
   to the address noted in the reply-to header (see 4.2.8) if it is
   present, else the From address (see 4.2.1) is used. The vCard EMAIL
   attribute, if present, SHOULD be the same as the reply-to address and
   may be the same as the From address.  While the vCard is the senders
   preferred address it SHOULD NOT be used to generate a reply.  Also,
   the Return-path address should not be used for replies.

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   Support of multiple originator header fields is often not possible on
   voice messaging systems, so it may be necessary to choose only one
   when gatewaying a VPIM message to another voice message system.
   However, implementers should note that this may make it impossible to
   send error messages and replies to their proper destinations.

   In some cases, a reply message is not possible, such as with a
   message created by telephone answering (i.e. classic voice mail).  In
   this case, the From field MUST contain the special address non-mail-
   user@domain (see 4.1.2).  A null ESMTP MAIL FROM address SHOULD also
   be used in this case (see 5.1.2).  A receiving VPIM system SHOULD NOT
   offer the user the option to reply to this kind of message.

4.7 Notification Messages

   VPIM delivery status notification messages (4.4.4) MUST be sent to
   the originator of the message when any form of non-delivery of the
   subject message or its components occurs.  These error messages must
   be sent to the return path (4.2.6) if present, otherwise, the From
   (4.2.1) address may be used.

   VPIM Receipt Notification messages (4.4.5) should be sent to the
   sender specified in the Disposition-Notification-To header field
   (4.2.16), only after the message has been presented to the recipient
   or if the message has somehow been disposed of without being
   presented to the recipient (e.g. if it were deleted before playing

   VPIM Notification messages may be positive or negative, and can
   indicate delivery at the server or receipt by the client.  However,
   the notification MUST be contained in a multipart/report container
   (4.4.3) and SHOULD contain a spoken error message.

   If a VPIM system receives a message with contents that are not
   understood (see 4.3 & 4.4), its handling is a local matter.  A
   delivery status notification SHOULD be generated if the message could
   not be delivered because of unknown contents (e.g., on traditional
   voice processing systems).  In some cases, the message may be
   delivered (with a positive DSN sent) to a mailbox before the
   determination of rendering can be made.

5. Message Transport Protocol

   Messages are transported between voice mail machines using the
   Internet Extended Simple Mail Transfer Protocol (ESMTP).  All
   information required for proper delivery of the message is included
   in the ESMTP dialog.  This information, including the sender and
   recipient addresses, is commonly referred to as the message

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   "envelope".  This information is equivalent to the message control
   block in many analog voice messaging  protocols.

   ESMTP is a general-purpose messaging protocol, designed both to send
   mail and to allow terminal console messaging.  Simple Mail Transport
   Protocol (SMTP) was originally created for the exchange of US-ASCII
   7-bit text messages.  Binary and 8-bit text messages have
   traditionally been transported by encoding the messages into a 7-bit
   text-like form.  [ESMTP] formalized an extension mechanism for SMTP,
   and subsequent RFCs have defined 8-bit text networking, command
   streaming, binary networking, and extensions to permit the
   declaration of message size for the efficient transmission of large
   messages such as multi-minute voice mail.

   The following sections list ESMTP commands, keywords, and parameters
   that are required and those that are optional for conformance to this

5.1 ESMTP Commands

5.1.1 HELO

   Base SMTP greeting and identification of sender.  This command is not
   to be sent by compliant systems unless the more-capable EHLO command
   is not accepted.  It is included for compatibility with general SMTP
   implementations.  Compliant servers MUST implement the HELO command
   for backward compatibility but clients SHOULD NOT send it unless EHLO
   is not supported.  From [SMTP]


   Originating mailbox.  This address contains the mailbox to which
   errors should be sent.  VPIM implementations SHOULD use the same
   address in the MAIL FROM command as is used in the From header field.
   This address is not necessarily the same as the message Sender listed
   in the message header fields if the message was received from a
   gateway or sent to an Internet-style mailing list. From [SMTP, ESMTP]

   The MAIL FROM address SHOULD be stored in the local message store for
   the purposes of generating a delivery status notification to the
   originator. The address indicated in the MAIL FROM command SHOULD be
   passed as a local system parameter or placed in a Return-Path: line
   inserted at the beginning of a VPIM message.  From [HOSTREQ]

   Since delivery status notifications MUST be sent to the MAIL FROM
   address, the use of the null address ("<>") is often used to prevent
   looping of messages.  This null address MAY be used to note that a
   particular message has no return path (e.g. a telephone answer

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   message).  From [SMTP]

5.1.3 RCPT TO

   Recipient's mailbox. The parameter to this command contains only the
   address to which the message should be delivered for this
   transaction.  It is the set of addresses in one or more RCPT TO
   commands that are used for mail routing. From [SMTP, ESMTP]

   Note: In the event that multiple transport connections to multiple
   destination machines are required for the same message, the set of
   addresses in a given transport connection may not match the list of
   recipients in the message header fields.

5.1.4 DATA

   Initiates the transfer of message data.  Support for this command is
   required.  Compliant implementations MUST implement the SMTP DATA
   command for backwards compatibility.  From [SMTP]

5.1.5 TURN

   Requests a change-of-roles, that is, the client that opened the
   connection offers to assume the role of server for any mail the
   remote machine may wish to send.  Because SMTP is not an
   authenticated protocol, the TURN command presents an opportunity to
   improperly fetch mail queued for another destination.  Compliant
   implementations SHOULD NOT implement the TURN command.  From [SMTP]

5.1.6 QUIT

   Requests that the connection be closed.  If accepted, the remote
   machine will reset and close the connection.  Compliant
   implementations MUST implement the QUIT command.  From [SMTP]

5.1.7 RSET

   Resets the connection to its initial state.  Compliant
   implementations MUST implement the RSET command. From [SMTP]

5.1.8 VRFY

   Requests verification that this node can reach the listed recipient.
   While this functionality is also included in the RCPT TO command,
   VRFY allows the query without beginning a mail transfer transaction.
   This command is useful for debugging and tracing problems.  Compliant
   implementations MAY implement the VRFY command.  From [SMTP] (Note
   that the implementation of VRFY may simplify the guessing of a

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   recipient's mailbox or automated sweeps for valid mailbox addresses,
   resulting in a possible reduction in privacy.  Various implementation
   techniques may be used to reduce the threat, such as limiting the
   number of queries per session.)  From [SMTP]

5.1.9 EHLO

   The enhanced mail greeting that enables a server to announce support
   for extended messaging options.  The extended messaging modes are
   discussed in subsequent sections of this document.  Compliant
   implementations MUST implement the ESMTP command and return the
   capabilities indicated later in this memo.  From [ESMTP]

5.1.10 BDAT

   The BDAT command provides a higher efficiency alternative to the
   earlier DATA command, especially for voice. The BDAT command provides
   for native binary transport of messages. Compliant implementations
   SHOULD support binary transport using the BDAT command [BINARY].

5.2 ESMTP Keywords

   The following ESMTP keywords indicate extended features useful for
   voice messaging.


   The "PIPELINING" keyword indicates ability of the receiving server to
   accept new commands before issuing a response to the previous
   command.  Pipelining commands dramatically improves performance by
   reducing the number of round-trip packet exchanges and makes it
   possible to validate all recipient addresses in one operation.
   Compliant implementations SHOULD support the command pipelining
   indicated by this keyword.  From [PIPE]

5.2.2 SIZE

   The "SIZE" keyword provides a mechanism by which the SMTP server can
   indicate the maximum size message supported.  Compliant servers MUST
   provide size extension to indicate the maximum size message that can
   be accepted.  Clients SHOULD NOT send messages larger than the size
   indicated by the server.  Clients SHOULD advertise SIZE= when sending
   messages to servers that indicate support for the SIZE extension.
   From [SIZE]

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   The "CHUNKING" keyword indicates that the receiver will support the
   high-performance binary transport mode.  Note that CHUNKING can be
   used with any message format and does not imply support for binary
   encoded messages. Compliant implementations MAY support binary
   transport indicated by this capability.  From [BINARY]


   The "BINARYMIME" keyword indicates that the SMTP server can accept
   binary encoded MIME messages. Compliant implementations MAY support
   binary transport indicated by this capability.  Note that support for
   this feature requires support of CHUNKING.  From [BINARY]

5.2.5 DSN

   The "DSN" keyword indicates that the SMTP server will accept explicit
   delivery status notification requests.  Compliant implementations
   MUST support the delivery notification extensions in [DRPT].


   The "ENHANCEDSTATUSCODES" keyword indicates that an SMTP server
   augments its responses with the enhanced mail system status codes
   [CODES].  These codes can then be used to provide more informative
   explanations of error conditions, especially in the context of the
   delivery status notifications format defined in [DSN]. Compliant
   implementations SHOULD support this capability.  From [STATUS]

5.3 ESMTP Parameters - MAIL FROM


   The current message is a binary encoded MIME messages.  Compliant
   implementations SHOULD support binary transport indicated by this
   parameter.  From [BINARY]

5.3.2 RET

   The RET parameter indicates whether the content of the message should
   be returned.  Compliant systems SHOULD honor a request for returned
   content. From [DRPT]

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5.3.3 ENVID

   The ENVID keyword of the SMTP MAIL command is used to specify an
   "envelope identifier" to be transmitted along with the message and
   included in any DSNs issued for any of the recipients named in this
   SMTP transaction.  The purpose of the envelope identifier is to allow
   the sender of a message to identify the transaction for which the DSN
   was issued. Compliant implementations MAY use this parameter.  From

5.4 ESMTP Parameters - RCPT TO

5.4.1 NOTIFY

   The NOTIFY parameter indicates the conditions under which a delivery
   report should be sent. Compliant implementations MUST honor this
   request.  From [DRPT]

5.4.2 ORCPT

   The ORCPT keyword of the RCPT command is used to specify an
   "original" recipient address that corresponds to the actual recipient
   to which the message is to be delivered.  If the ORCPT esmtp-keyword
   is used, it MUST have an associated esmtp-value, which consists of
   the original recipient address, encoded according to the rules below.
   Compliant implementations MAY use this parameter.  From [DRPT]

5.5 ESMTP - SMTP Downgrading

   The ESMTP extensions suggested or required for conformance to VPIM
   fall into two categories.  The first category includes features which
   increase the efficiency of the transport system such as SIZE,
   BINARYMIME, and PIPELINING.  In the event of a downgrade to a less
   functional transport system, these features can be dropped with no
   functional change to the sender or recipient.

   The second category of features are transport extensions in support
   of new functions.  DSN and EnhancedStatusCodes provide essential
   improvements in the handling of delivery status notifications to
   bring email to the level of reliability expected of Voice Mail.  To
   ensure a consistent level of service across an intranet or the global
   Internet, it is essential that VPIM compliant ESMTP support the ESMTP
   DSN extension at all hops between a VPIM originating system and the
   recipient system. In the situation where a `downgrade' is unavoidable
   a relay hop may be forced (by the next hop) to forward a VPIM message
   without the ESMTP request for positive delivery status notification.
   It is RECOMMENDED that the downgrading system should continue to
   attempt to deliver the message, but MUST send an appropriate delivery

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   notification to the originator, e.g. the message left an ESMTP host
   and was sent (unreliably) via SMTP.

(page 30 continued on part 2)

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