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RFC 0976

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UUCP mail interchange format standard

Updated by:    1137

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Network Working Group                                    Mark. R. Horton
Request for Comments: 976                              Bell Laboratories
                                                           February 1986

                 UUCP Mail Interchange Format Standard

Status of This Memo

   In response to the need for maintenance of current information about
   the status and progress of various projects in the ARPA-Internet
   community, this RFC is issued for the benefit of community members.
   The information contained in this document is accurate as of the date
   of publication, but is subject to change. Subsequent RFCs will
   reflect such changes.

   This document defines the standard format for the transmission of
   mail messages between machines in the UUCP Project.  It does not
   address the format for storage of messages on one machine, nor the
   lower level transport mechanisms used to get the data from one
   machine to the next.  It represents a standard for conformance by
   hosts in the UUCP zone.  Distribution of this memo is unlimited.

1.  Introduction

   This document is intended to define the standard format for the
   transmission of mail messages between machines in the UUCP Project.
   It does not address the format for storage of messages on one
   machine, nor the lower level transport mechanisms used to get the
   data from one machine to the next.  We assume remote execution of the
   rmail command (or equivalent) as the UUCP network primitive

   The general philosophy is that, if we were to invent a new standard,
   we would make ourselves incompatible with existing systems.  There
   are already too many (incompatible) standards in the world, resulting
   in ambiguities such as a!b@c.d which is parsed a!(b@c.d) in the old
   UUCP world, and (a!b)@c.d in the Internet world.  (Neither standard
   allows parentheses, and in adding them we would be compatible with
   neither.  There would also be serious problems with the shell and
   with the UUCP transport mechanism.)

   Having an established, well documented, and extensible family of
   standards already defined by the ARPA community, we choose to adopt
   these standards for the UUCP zone as well.  (The UUCP zone is that
   subset of the community connected by UUCP which chooses to register
   with the UUCP project.  It represents an administrative entity.)
   While the actual transport mechanism is up to the two hosts to
   arrange, and might include UUCP, SMTP, MMDF, or some other facility,
   we adopt RFC-920 (domains) and RFC-822 (mail format) as UUCP zone
   standards.  All mail transmitted between systems should conform to

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   those two standards.  In addition, should the ARPA community change
   these standards at a later time, we intend to change our standards to
   remain compatible with theirs, given a reasonable time to upgrade

   This document specifies an interpretation of RFC-822 and RFC-920 in
   the UUCP world.  It shows how the envelope should be encoded, and how
   UUCP routing is accomplished in an environment of mixed

2.  Basics

   Messages can be divided into two parts: the envelope and the message.
   The envelope contains information needed by the mail transport
   services, and the message contains information useful to the sender
   and receiver.  The message is divided into the header and the body.
   Sometimes an intermediate host will add to the message (e.g. a
   Received line) but, except in the case of a gateway which must
   translate formats, it is not expected that intermediate hosts will
   change the message itself.  In the UUCP world, the envelope consists
   of the "destination addresses" (normally represented as the argument
   or arguments to the rmail command) and the "source path" (normally
   represented in one or more lines at the beginning of the message
   beginning either "From " or ">From ", sometimes called "From_
   lines".)  The RFC-822 header lines (including "From:" and "To:") are
   part of the message, as is the text of the message body itself.

   UUCP uses short host names, such as "ucbvax", at and below the
   transport layer.  We refer to these names as "6 letter names",
   because all implementations of UUCP consider at least the first 6
   letters significant.  (Some consider the first 7 or the first 14
   significant, but we must use the lowest common denominator.) UUCP
   names may be longer than 6 characters, but all such names much be
   unique in their first 6 letters.  RFC-920 domain names, such as
   "ucbvax.Berkeley.EDU", are called "domain names." The two names are
   different.  Upper and lower case are usually considered different in
   6 letter names, but are considered equivalent in domain names.  Names
   such as "ucbvax.UUCP", consisting of a 6 letter name followed by
   ".UUCP", previously were domain style references to a host with a
   given 6 letter name.  Such names are being phased out in favor of
   organizational domain names such as "ucbvax.Berkeley.EDU"

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2.1  Hybrid Addresses

   There are (among others) two major kinds of mailing address syntax
   used in the UUCP world.  The a!b!c!user ("bang paths") is used by
   older UUCP software to explicitly route mail to the destination.  The
   user@domain ("domain") syntax is used in conformance to RFC-822.
   Under most circumstances, it is possible to look at a given address
   and determine which sort of address it is.  However, a hybrid address
   with a ! to the left of an @, such as a!b@c, is ambiguous: it could
   be interpreted as (a!b)@c.d or a!(b@c.d).  Both interpretations can
   be useful.  The first interpretation is required by RFC-822, the
   second is a de-facto standard in the UUCP software.

   Because of the confusion surrounding hybrid addresses, we recommend
   that all transport layer software avoid the use of hybrid addresses
   at all times.  A pure bang syntax can be used to disambiguate, being
   written c.d!a!b in the first case above, and a!c.d!b in the second.
   We recommend that all implementations use this "bang domain" syntax
   unless they are sure of what is running on the next machine.

   In conformance with RFC-822 and the AT&T Message Transfer
   Architecture, we recommand that any host that accepts hybrid
   addresses apply the (a!b)@c.d interpretation.

2.2  Transport

   Since SMTP is not available to much of the UUCP domain, we define the
   method to be used for "remote execution" based transport mechanisms.
   The command to be "remotely executed" should read

      rmail user@domain ...

   with the message on the standard input of the command.  The
   "user@domain" argument must conform to RFC-920 and RFC-822.  More
   than one address argument is allowed, in order to save transmission
   costs for multiple recipients of the same message.

   An alternative form that may be used is

      rmail domain!user

   where "domain" contains at least one period and no !'s.  This is to
   be interpreted exactly the same as user@domain, and can be used to
   transport a message across old UUCP hosts without fear that they
   might change the address.  The "user" string can contain any
   characters except "@".  This character is forbidden because it is
   unknown what an intermediate host might do to it. (It is also

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   recommended that the "%" character be avoided, since some hosts treat
   "%" as a synonym for "@".) However, to route across hosts that don't
   understand domains, the following is possible

      rmail a!b!c!domain!user

   A "domain" can be distinguished from a 6 letter UUCP site name
   because a domain will contain at least one period.  (In the case of
   single level domains with no periods, a period should be added to the
   end, e.g. Mark.Horton@att becomes "att.!Mark.Horton".  A translator
   from ! to @ format should remove a trailing dot at the end of the
   domain, if one is present.) We don't expect this to happen, except
   for local networks using addresses like "user@host".

   A simple implementation can always generate domain!user syntax
   (rather than user@domain) since it is safe to assume that gateways
   are class 3 (Classes are explained in section 3.5).

2.3  Batch SMTP

   Standard conforming implementations may optionally support a protocol
   called "Batch SMTP".  SMTP (Simple Mail Transfer Protocol) is the
   ARPA community standard mail transfer protocol (RFC-821). It is also
   used on BITNET and Mailnet.  While SMTP was designed to be
   interactive, it is possible to batch up a series of commands and send
   them off to a remote machine for batch execution.  This is used on
   BITNET, and is appropriate for UUCP.  One advantage to BSMTP is that
   the UNIX shell does not get involved in the interpretation of
   messages, so it becomes possible to include special characters such
   as space and parentheses in electronic messages.  (Such characters
   are expected to be popular in X.400 addresses.)

   To support BSMTP on UNIX, a conforming host should arrange that mail
   to the user "b-smtp" is interpreted as Batch SMTP commands.  (We use
   b-smtp instead of bsmtp because bsmtp might conflict with a login
   name.) Since many mail systems treat lines consisting of a single
   period as an "end of file" flag, and since SMTP uses the period as a
   required end of file flag, and to strip off headers, we put an extra
   "#" at the beginning of each BSMTP line.  On a sendmail system, an
   easy way to implement this is to include the alias

      b-smtp: "|egrep '^#' | sed 's/^#//' | /usr/lib/sendmail -bs"

   which will feed the commands to an SMTP interpreter.  A better
   solution would appropriately check for errors and send back an error
   message to the sender.

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   An example BSMTP message from seismo.CSS.GOV to cbosgd.ATT.COM is
   shown here.  This sample is the file shipped over the UUCP link for
   in put to the command "rmail b-smtp".  Note that the RFC- 822 message
   is between the DATA line and the period line.  The envelope
   information is passed in the MAIL FROM and RCPT TO lines.  The name
   of the sending system is in the HELO line.  The actual envelope
   information (above the # lines) is ignored and need not be present.

      From foo!bar Sun Jan 12 23:59:00 1986 remote from seismo Date:
      Tue, 18 Feb 86 13:07:36 EST
      From: mark@ucbvax.Berkeley.EDU
      Message-Id: <8602181807.AA10228@mark@ucbvax.Berkeley.EDU> To:

      #HELO seismo.CSS.GOV
      #MAIL FROM:<mark@ucbvax.Berkeley.EDU>
      #RCPT TO:<mark@cbosgd.ATT.COM>
      #Date: Tue, 18 Feb 86 13:07:36 EST
      #From: mark@ucbvax.Berkeley.EDU
      #Message-Id: <8602181807.AA10228@mark@ucbvax.Berkeley.EDU> #To:
      #This is a sample message.

2.4  Envelope

   The standard input of the command should begin with a single line

      From domain!user date remote from system

   followed immediately by the RFC-822 format headers and body of the
   message.  It is possible that there will be additional From_ lines
   preceding this line - these lines may be added, one line for each
   system the message passes through.  It is also possible that the
   "system" fields will be stacked into a single line, with many !'s in
   the "user" string.  The ">" character may precede the "From".  In
   general, this is the "envelope" information, and should follow the
   same conventions that previous UUCP mail has followed.  The primary
   difference is that, when the system names are stacked up, if
   previously the result would have been a!b!c!mysys!me, the new result
   will be a!b!c!mysys!domain!me, where domain will contain at least one
   period, and "mysys" is often the 6 letter UUCP name for the same

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   system named by "domain".  If the "domain!" is redundant, it may be
   omitted from the envelope, either in the source path or in the
   destination address.

   The receiving system may discard extra "From_" lines if it folds the
   information into a a single From_ line. It passes the
   path!domain!user along as the "envelope" information containing the
   address of the sender of the message, and possibly preserves the
   forwarding date and system in a newly generated header line, such as
   Received or Sent-By.  (Adding Received using this information is
   discouraged, since the line appears to have been added on a different
   system than the one actually adding it.  That other system may have
   actually included a Received line too! The Sent-By line is similar to
   Received, but the date need not be converted into RFC-822 format, and
   the line is not claimed to have been added by the system whose name
   is mentioned.)

   If the receiving system passes the message along to another system,
   it will add a "From_" line to the front, giving the same user@domain
   address for the sender, and its own name for the system.  If the
   receiving system stores the message in a local mailbox, it is
   recommended that a single "From_" line be generated at the front of
   the message, keeping the date (in the same format, since certain mail
   reading programs are sensitive to this format), and not using the
   "remote from system" syntax.

   Note - if an intermediate system adds text such as "system!" to the
   front of a "user@domain" syntax address, either in the envelope or
   the body, this is a violation of this standard and of RFC-822.

2.5  Routing

   In order to properly route mail, it is sometimes necessary to know
   what software a destination or intermediate machine is running, or
   what conventions it follows.  We have tried to minimize the amount of
   this information that is necessary, but the support of subdomains may
   require that different methods are used in different situations.  For
   purposes of predicting the behavior of other hosts, we divide hosts
   into three classes. These classes are:

   Class 1   old-style UUCP ! routing only.  We assume that the host
             understands local user names:

                  rmail user

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             and bang paths

                  rmail host1!host2!user

             but we assume nothing more about the host.  If we have
             no information about a host, we can treat it as class 1
             with no problems, since we make no assumptions about
             how it will handle hybrid addresses.

   Class 2   Old style UUCP ! routing, and 4.2BSD style domain
             parsing.  We assume the capabilities of class 1, plus
             the ability to understand

                  rmail user@domain

             if the "domain" is one outside the UUCP zone which
             the host knows about.  Class 2 hosts do not necessarily
             understand domain!user or have routers.  Hosts in non-

             UUCP RFC-920 domains are considered class 2, even though
             they may not understand host!user.

   Class 3   All class 1 and 2 features are present.  In addition,
             class 3 hosts must be able to route UUCP mail for hosts
             that are not immediately adjacent and also understand
             the syntax

                  rmail domain!user

             as described above.  All gateways into UUCP must be
             class 3.

   This document describes what class 3 hosts must be able to process.
   Classes 1 and 2 already exist, and will continue to exist for a long
   time, but are viewed as "older systems" that may eventually be
   upgraded to class 3 status.

3.  Algorithm

   The algorithm for delivering a message to an address "user@domain"
   over UUCP links can be summarized as follows:

      a.  If the address is actually of the form @domain1:user@domain2,
          the "domain" used for the remainder should be "domain1"
          instead of "domain2", and the bang form reads

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      b.  Determine d: the most specific part of "domain" that is
          recognized locally.  This part will be a suffix of "domain".
          This can be done by scanning through a table with entries that
          go from specific to general, comparing entries with "domain"
          to see if the entries are at the tail of "domain".  For
          example, with the address "", if the local
          host recognizes "uucp" and "", d would be "".
          The final entry in the table will be the null string, matching
          any completely unrecognized domain.

      c.  Look in the found table entry for g: the name of the
          "gateway", and for r: a UUCP !-style route to reach g.  G is
          not necessarily directly connected to the local host, but
          should be viewed as a gateway into the d domain.  (The values
          of g and r for a given d may be different on different hosts,
          although g will often be the same.)

      d.  Look at the beginning of r to find the "next hop" host n. N
          will always be directly connected to the local host.

      e.  Determine, if possible, the class of g and n.

      f.  Create an appropriate destination string s to be interpreted
          by n.  (See below.)

      g.  Pass the message off to n with destination information s.

      In an environment with other types of networks that do not use
      UUCP !  parsing, the table will probably contain additional
      information, such as which type of link to use.  The path
      information may be replaced in other environments by information
      specific to the network.

      The first entries in the table mentioned in part (b) are normally
      very specific, and allow well known routes to be constructed
      directly instead of routing through the domain tree.  The domain
      tree should be reserved for cases where no better information is
      available, or where traffic is very light, or where the default
      route is the best available.  If a better route is available, that
      information can be put in the table.  If a host has any
      significant amount of traffic sent to a second host, it is
      normally expected that the two hosts will set up a direct UUCP
      link and make an entry in their tables to send mail directly, even
      if they are in separate domains.  Routing tables should be
      constructed to try to keep paths short and inexpensive for as much
      traffic as possible.

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      Here are some hints for the construction of the destination string
      n (step f above.) The "envelope recipient" information (the
      argument(s) to rmail) may be in either domain ! form
      (!user) or domain @ form ( as long as the
      sending site is sure the next hop is class 3.  If the next hop is
      not class 3, or the sending site is not sure, the ! form should be
      used, if possible, since it is hard to predict what the next hop
      would do with a hybrid address.

      If the gateway is known to be class 3, domain ! form may be used,
      but if the sending site is not sure, and the entire destination
      string was matched in the lookup (rather than some parent domain),
      the 6 letter ! form should be used: r!user, for example:
      dumbhost!host!user.  If the gateway appears to actually be a
      gateway for a subdomain, e.g. because a parent domain was matched,
      (such as the address, where
      was not found but was) it can be assumed to be at
      class 3.  This allows routes such as
      dumbhost!domain!!user to be used with a reasonable
      degree of safety.  If a direct link exists to the destination
      host, the user@domain syntax or the domain!user syntax may be

      All hosts conforming to this standard are class 3, and all
      subdomain gateways must be class 3 hosts.

4.  Example

   Suppose host A.D.COM sends mail to host C.D.COM.  Let's suppose that
   the 6 letter names for these hosts are aname and dname, and that the
   intermediate host to be routed through has name bname.

   The user on A types


   The user interface creates a file such as

      Date:  9 Jan 1985   8:39 EST
      From: myname@A.D.COM (My Name)
      Subject: sample message

      This is a sample message

   and passes it to the transport mechanism with a command such as

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      sendmail < file

   The transport mechanism looks up a route to  It does not
   find in its database, so it looks up, and finds that
   the path is bname!dname!%s, and that is a class 3 host.
   Plugging in!user, it gets the path bname!dname!!user.
   (If it had found with path bname!cname!%s, it would have
   omitted the domain from the resulting path: bname!cname!user, since
   it is not sure whether the destination host is class 1, 2, or 3.)

   It prepends a From_ line and passes it to uux:

      uux - bname!rmail dname!!user < file2

   where file2 contains

      From A.D.COM!user Wed Jan  9 12:43:35 1985 remote from aname Date:
      9 Jan 1985   8:39 EST
      From: myname@A.D.COM (My Name)
      Subject: sample message

      This is a sample message

   (Note the blank line at the end of the message - at least one blank
   line is required.) This results in the command

      rmail dname!!user

   running on B.  B prepends its own from line and passes the mail

      uux - dname!rmail!user < file3

   where file3 contains

      From nuucp Wed Jan  9 12:43:35 1985 remote from bname >From
      A.D.COM!user Wed Jan  9 11:21:48 1985 remote from aname Date:  9
      Jan 1985   8:39 EST
      From: myname@A.D.COM (My Name)
      Subject: sample message

      This is a sample message

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   The command


   is run on C, which stacks the From_ lines

      From bname!aname!A.D.COM!user Wed Jan  9 12:43:35 1985 Date:  9
      Jan 1985   8:39 EST
      From: myname@A.D.COM (My Name)
      Subject: sample message

      This is a sample message

   and stores the message locally, probably in this same format.

5.  Summary

   Hosts conforming to this standard should accept all of the following

      rmail localuser               (no !%@ in user)
      rmail hosta!hostb!user        (no !%@ in user)
      rmail user@domain             (only . in domain)
      rmail domain!user             (at least 1 . in domain)
      rmail domain.!user            (in case domain has no dots)

   The "envelope" portion of the message ("From_" lines) should conform
   to existing conventions, using ! routing.  The "heading" portion of
   the message (the Word: lines such as Date:, From:, To:, and Subject:)
   must conform to RFC-822.  All header addresses must be in the @ form.
   The originating site should ensure that the addresses conform to
   RFC-822, since no requirement is placed on forwarding sites or
   gateways to transform addresses into legal RFC-822 format.  (Such
   forwarding sites and gateways should NOT, however, change a legal
   RFC-822 address such as user@domain into an illegal RFC-822 address
   such as gateway!user@domain, even if forwarding to a class 1 UUCP

6.  References

   [1]  Postel, J., "Simple Mail Transfer Protocol", RFC-821,
        USC/Information Sciences Institute, August, 1982.

   [2]  Crocker, D., "Standard for the Format of ARPA Internet Text
        Messages", RFC-822, Department of Electrical Engineering,
        University of Delaware, August, 1982.

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   [3]  Postel, J., and J. K. Reynolds, "Domain Requirements", RFC-920,
        USC/Information Sciences Institute, October, 1984.