The forms of identity used by Internet Mail are: mailbox, domain
name, message-ID, and ENVID (envelope identifier). Each is globally
"A mailbox receives mail. It is a conceptual entity that does not
necessarily pertain to file storage." [RFC5322]
A mailbox is specified as an Internet Mail address <addr-spec>. It
has two distinct parts, separated by an at-sign (@). The right side
is a globally interpreted domain name associated with an ADMD.
Domain names are discussed in Section 3.3. Formal Internet Mail
addressing syntax can support source routes to indicate the path
through which a message ought to be sent. The use of source routes
is not common and has been deprecated in [RFC5321].
The portion to the left of the at-sign contains a string that is
globally opaque and is called the <local-part>. It is interpreted
only by the entity specified by the address's domain name. Except as
noted later in this section, all other entities treat the
<local-part> as an uninterpreted literal string and preserve all
of its original details. As such, its public distribution is
equivalent to sending a Web browser "cookie" that is only interpreted
upon being returned to its creator.
Some local-part values have been standardized for contacting
personnel at an organization. These names cover common operations
and business functions [RFC2142].
It is common for sites to have local structuring conventions for the
left-hand side, <local-part>, of an <addr-spec>. This permits sub-
addressing, such as for distinguishing different discussion groups
used by the same participant. However, it is worth stressing that
these conventions are strictly private to the User's organization and
are not interpreted by any domain except the one listed in the right
side of the <addr-spec>. The exceptions are those specialized
services that conform to public, standardized conventions, as noted
Basic email addressing defines the <local-part> as being globally
opaque. However, there are some uses of email that add a
standardized, global schema to the value, such as between an Author
and a Gateway. The <local-part> details remain invisible to the
public email transfer infrastructure, but provide addressing and
handling instructions for further processing by the Gateway.
Standardized examples of these conventions are the telephone
numbering formats for the Voice Profile for Internet Mail (VPIM)
[RFC3801], such as:
and iFax ([RFC3192], [RFC4143] such as:
3.2. Scope of Email Address Use
Email addresses are being used far beyond their original role in
email transfer and delivery. In practical terms, an email address
string has become the common identifier for representing online
identity. Hence, it is essential to be clear about both the nature
and role of an identity string in a particular context and the entity
responsible for setting that string. For example, see Sections
4.1.4, 4.3.3, and 5.
3.3. Domain Names
A domain name is a global reference to an Internet resource, such as
a host, a service, or a network. A domain name usually maps to one
or more IP Addresses. Conceptually, the name can encompass an
organization, a collection of machines integrated into a homogeneous
service, or a single machine. A domain name can be administered to
refer to an individual User, but this is not common practice. The
name is structured as a hierarchical sequence of labels, separated by
dots (.), with the top of the hierarchy being on the right end of the
sequence. There can be many names in the sequence -- that is, the
depth of the hierarchy can be substantial. Domain names are defined
and operated through the Domain Name System (DNS) ([RFC1034],
When not part of a mailbox address, a domain name is used in Internet
Mail to refer to the ADMD or to the host that took action upon the
message, such as providing the administrative scope for a message
identifier or performing transfer processing.
3.4. Message Identifier
There are two standardized tags for identifying messages: Message-ID:
and ENVID. A Message-ID: pertains to content, and an ENVID pertains
IMF provides for, at most, a single Message-ID:. The Message-ID: for
a single message, which is a user-level IMF tag, has a variety of
uses including threading, aiding identification of duplicates, and
DSN (Delivery Status Notification) tracking. The Originator assigns
the Message-ID:. The Recipient's ADMD is the intended consumer of
the Message-ID:, although any Actor along the transfer path can use
Message-ID: is globally unique. Its format is similar to that of a
mailbox, with two distinct parts separated by an at-sign (@).
Typically, the right side specifies the ADMD or host that assigns the
identifier, and the left side contains a string that is globally
opaque and serves to uniquely identify the message within the domain
referenced on the right side. The duration of uniqueness for the
message identifier is undefined.
When a message is revised in any way, the decision whether to assign
a new Message-ID: requires a subjective assessment to determine
whether the editorial content has been changed enough to constitute a
new message. [RFC5322] states that "a message identifier pertains to
exactly one version of a particular message; subsequent revisions to
the message each receive new message identifiers." Yet experience
suggests that some flexibility is needed. An impossible test is
whether the Recipient will consider the new message to be equivalent
to the old one. For most components of Internet Mail, there is no
way to predict a specific Recipient's preferences on this matter.
Both creating and failing to create a new Message-ID: have their
Here are some guidelines and examples:
o If a message is changed only in form, such as character encoding,
it is still the same message.
o If a message has minor additions to the content, such as a Mailing
List tag at the beginning of the RFC5322.Subject header field, or
some Mailing List administrative information added to the end of
the primary body part text, it is probably the same message.
o If a message has viruses deleted from it, it is probably the same
o If a message has offensive words deleted from it, some Recipients
will consider it the same message, but some will not.
o If a message is translated into a different language, some
Recipients will consider it the same message, but some will not.
o If a message is included in a digest of messages, the digest
constitutes a new message.
o If a message is forwarded by a Recipient, what is forwarded is a
o If a message is "redirected", such as using IMF "Resent-*" header
fields, some Recipients will consider it the same message, but
some will not.
The absence of both objective, precise criteria for regenerating a
Message-ID: and strong protection associated with the string means
that the presence of an ID can permit an assessment that is
marginally better than a heuristic, but the ID certainly has no value
on its own for strict formal reference or comparison. For that
reason, the Message-ID: is not intended to be used for any function
that has security implications.
The ENVID (envelope identifier) can be used for message-tracking
purposes ([RFC3885], [RFC3464]) concerning a single posting/delivery
transfer. The ENVID labels a single transit of the MHS by a specific
message. So, the ENVID is used for one message posting until that
message is delivered. A re-posting of the message, such as by a
Mediator, does not reuse that ENVID, but can use a new one, even
though the message might legitimately retain its original
The format of an ENVID is free form. Although its creator might
choose to impose structure on the string, none is imposed by Internet
standards. By implication, the scope of the string is defined by the
domain name of the Return Address.
4. Services and Standards
The Internet Mail architecture comprises six basic types of
functionality, which are arranged to support a store-and-forward
service. As shown in Figure 5, each type can have multiple
instances, some of which represent specialized roles. This section
considers the activities and relationships among these components,
and the Internet Mail standards that apply to them.
Message User Agent (MUA)
Author MUA (aMUA)
Recipient MUA (rMUA)
Message Submission Agent (MSA)
Author-focused MSA functions (aMSA)
MHS-focused MSA functions (hMSA)
Message Transfer Agent (MTA)
Message Delivery Agent (MDA)
Recipient-focused MDA functions (rMDA)
MHS-focused MDA functions (hMDA)
... lines indicate supporting transfers or roles
*** lines indicate aggregated service
Figure 5: Protocols and Services4.1. Message Data
The purpose of the Message Handling System (MHS) is to exchange an
IMF message object among participants [RFC5322]. All of its
underlying mechanisms serve to deliver that message from its Author
to its Recipients. A message can be explicitly labeled as to its
A message comprises a transit-handling envelope and the message
content. The envelope contains information used by the MHS. The
content is divided into a structured header and the body. The header
comprises transit-handling trace information and structured fields
that are part of the Author's message content. The body can be
unstructured lines of text or a tree of multimedia subordinate
objects, called "body-parts" or, popularly, "attachments".
[RFC2045], [RFC2046], [RFC2047], [RFC4288], [RFC4289], [RFC2049].
In addition, Internet Mail has a few conventions for special control
Delivery Status Notification (DSN):
A Delivery Status Notification (DSN) is a message that can be
generated by the MHS (MSA, MTA, or MDA) and sent to the
RFC5321.MailFrom address. MDA and MTA are shown as sources of
DSNs in Figure 5, and the destination is shown as Returns. DSNs
provide information about message transit, such as transfer errors
or successful delivery [RFC3461].
Message Disposition Notification (MDN):
A Message Disposition Notification (MDN) is a message that
provides information about post-delivery processing, such as
indicating that the message has been displayed [RFC3798] or the
form of content that can be supported [RFC3297]. It can be
generated by an rMUA and is sent to the
Disposition-Notification-To addresses. The mailbox for this is
shown as Disp in Figure 5.
Message Filtering (SIEVE):
Sieve is a scripting language used to specify conditions for
differential handling of mail, typically at the time of delivery
[RFC5228]. Scripts can be conveyed in a variety of ways, such as
a MIME part in a message. Figure 5 shows a Sieve script going
from the rMUA to the MDA. However, filtering can be done at many
different points along the transit path, and any one or more of
them might be subject to Sieve directives, especially within a
single ADMD. Figure 5 shows only one relationship, for (relative)
Internet Mail has a fragmented framework for transit-related handling
information. Information that is used directly by the MHS is called
the "envelope". It directs handling activities by the transfer
service and is carried in transfer-service commands. That is, the
envelope exists in the transfer protocol SMTP [RFC5321].
Trace information, such as RFC5322.Received, is recorded in the
message header and is not subsequently altered [RFC5322].
4.1.2. Header Fields
Header fields are attribute name/value pairs that cover an extensible
range of email-service parameters, structured user content, and user
transaction meta-information. The core set of header fields is
defined in [RFC5322]. It is common practice to extend this set for
different applications. Procedures for registering header fields are
defined in [RFC3864]. An extensive set of existing header field
registrations is provided in [RFC4021].
One danger of placing additional information in header fields is that
Gateways often alter or delete them.
The body of a message might be lines of ASCII text or a
hierarchically structured composition of multimedia body part
attachments using MIME ([RFC2045], [RFC2046], [RFC2047], [RFC4288],
4.1.4. Identity References in a Message
Table 1 lists the core identifiers present in a message during
| Layer | Field | Set By |
| Message Body | MIME Header | Author |
| Message header | From: | Author |
| fields | | |
| | Sender: | Originator |
| | Reply-To: | Author |
| | To:, CC:, BCC: | Author |
| | Message-ID: | Originator |
| | Received: | Originator, Relay, |
| | | Receiver |
| | Return-Path: | MDA, from MailFrom |
| | Resent-*: | Mediator |
| | List-Id: | Mediator |
| | List-*: | Mediator |
| SMTP | HELO/EHLO | Latest Relay Client |
| | ENVID | Originator |
| | MailFrom | Originator |
| | RcptTo | Author |
| | ORCPT | Originator |
| IP | Source Address | Latest Relay Client |
Layer - The part of the email architecture that uses the
Field - The protocol construct that contains the identifier.
Set By - The Actor role responsible for specifying the identifier
value (and this can be different from the Actor that performs the
fill-in function for the protocol construct).
Table 1: Layered Identities
These are the most common address-related fields:
RFC5322.From: Set by - Author
Names and addresses for Authors of the message content are listed
in the From: field.
RFC5322.Reply-To: Set by - Author
If a Recipient sends a reply message that would otherwise use the
RFC5322.From field addresses in the original message, the
addresses in the RFC5322.Reply-To field are used instead. In
other words, this field overrides the From: field for responses
RFC5322.Sender: Set by - Originator
This field specifies the address responsible for submitting the
message to the transfer service. This field can be omitted if it
contains the same address as RFC5322.From. However, omitting this
field does not mean that no Sender is specified; it means that
that header field is virtual and that the address in the From:
field is to be used.
Specification of the notifications Return Addresses, which are
contained in RFC5321.MailFrom, is made by the RFC5322.Sender.
Typically, the Return address is the same as the Sender address.
However, some usage scenarios require it to be different.
RFC5322.To/.CC: Set by - Author
These fields specify MUA Recipient addresses. However, some or
all of the addresses in these fields might not be present in the
The distinction between To and CC is subjective. Generally, a To
addressee is considered primary and is expected to take action on
the message. A CC addressee typically receives a copy as a
RFC5322.BCC: Set by - Author
A copy of the message might be sent to an addressee whose
participation is not to be disclosed to the RFC5322.To or
RFC5322.CC Recipients and, usually, not to the other BCC
Recipients. The BCC: header field indicates a message copy to
such a Recipient. Use of this field is discussed in [RFC5322].
RFC5321.HELO/.EHLO: Set by - Originator, MSA, MTA
Any SMTP client -- including Originator, MSA, or MTA -- can
specify its hosting domain identity for the SMTP HELO or EHLO
RFC3461.ENVID: Set by - Originator
The MSA can specify an opaque string, to be included in a DSN, as
a means of assisting the Return Address Recipient in identifying
the message that produced a DSN or message tracking.
RFC5321.MailFrom: Set by - Originator
This field is an end-to-end string that specifies an email address
for receiving return control information, such as returned
messages. The name of this field is misleading, because it is not
required to specify either the Author or the Actor responsible for
submitting the message. Rather, the Actor responsible for
submission specifies the RFC5321.MailFrom address. Ultimately,
the simple basis for deciding which address needs to be in the
RFC5321.MailFrom field is to determine which address is to be
informed about transfer-level problems (and possibly successes).
RFC5321.RcptTo: Set by - Author, Final MTA, MDA
This field specifies the MUA mailbox address of a Recipient. The
string might not be visible in the message content header. For
example, the IMF destination address header fields, such as
RFC5322.To, might specify a Mailing List mailbox, while the
RFC5321.RcptTo address specifies a member of that list.
RFC5321.ORCPT: Set by - Originator.
This is an optional parameter to the RCPT command, indicating the
original address to which the current RCPT TO address corresponds,
after a mapping was performed during transit. An ORCPT is the
only reliable way to correlate a DSN from a multi-Recipient
message transfer with the intended Recipient.
RFC5321.Received: Set by - Originator, Relay, Mediator, Dest
This field contains trace information, including originating host,
Relays, Mediators, and MSA host domain names and/or IP Addresses.
RFC5321.Return-Path: Set by - Originator
The MDA records the RFC5321.MailFrom address into the
RFC2919.List-Id: Set by - Mediator, Author
This field provides a globally unique Mailing List naming
framework that is independent of particular hosts [RFC2919].
The identifier is in the form of a domain name; however, the
string usually is constructed by combining the two parts of an
email address. The result is rarely a true domain name, listed in
the domain name service, although it can be.
RFC2369.List-*: Set by - Mediator, Author
[RFC2369] defines a collection of message header fields for use by
Mailing Lists. In effect, they supply list-specific parameters
for common Mailing-List user operations. The identifiers for
these operations are for the list itself and the user-as-
RFC0791.SourceAddr: Set by - The Client SMTP sending host
immediately preceding the current receiving SMTP server
[RFC0791] defines the basic unit of data transfer for the
Internet: the IP datagram. It contains a Source Address field
that specifies the IP Address for the host (interface) from which
the datagram was sent. This information is set and provided by
the IP layer, which makes it independent of mail-level mechanisms.
As such, it is often taken to be authoritative, although it is
possible to provide false addresses.
4.2. User-Level Services
Interactions at the user level entail protocol exchanges, distinct
from those that occur at lower layers of the Internet Mail MHS
architecture that is, in turn, above the Internet Transport layer.
Because the motivation for email, and much of its use, is for
interaction among people, the nature and details of these protocol
exchanges often are determined by the needs of interpersonal and
group communication. To accommodate the idiosyncratic behavior
inherent in such communication, only subjective guidelines, rather
than strict rules, can be offered for some aspects of system
behavior. Mailing Lists provide particularly salient examples.
4.2.1. Message User Agent (MUA)
A Message User Agent (MUA) works on behalf of User Actors and User
applications. It is their representative within the email service.
The Author MUA (aMUA) creates a message and performs initial
submission into the transfer infrastructure via a Mail Submission
Agent (MSA). It can also perform any creation- and posting-time
archiving in its Message Store (aMS). An MUA aMS can organize
messages in many different ways. A common model uses aggregations,
called "folders"; in IMAP they are called "mailboxes". This model
allows a folder for messages under development (Drafts), a folder for
messages waiting to be sent (Queued or Unsent), and a folder for
messages that have been successfully posted for transfer (Sent). But
none of these folders is required. For example, IMAP allows drafts
to be stored in any folder, so no Drafts folder needs to be present.
The Recipient MUA (rMUA) works on behalf of the Recipient to process
received mail. This processing includes generating user-level
disposition control messages, displaying and disposing of the
received message, and closing or expanding the user-communication
loop by initiating replies and forwarding new messages.
NOTE: Although not shown in Figure 5, an MUA itself can have a
distributed implementation, such as a "thin" user-interface
module on a constrained device such as a smartphone, with
most of the MUA functionality running remotely on a more
capable server. An example of such an architecture might use
IMAP [RFC3501] for most of the interactions between an MUA
client and an MUA server. An approach for such scenarios is
defined by [RFC4550].
A Mediator is a special class of MUA. It performs message
re-posting, as discussed in Section 2.1.
An MUA can be automated, on behalf of a User who is not present at
the time the MUA is active. One example is a bulk sending service
that has a timed-initiation feature. These services are not to be
confused with a Mailing List Mediator, since there is no incoming
message triggering the activity of the automated service.
A popular and problematic MUA is an automatic responder, such as one
that sends out-of-office notices. This behavior might be confused
with that of a Mediator, but this MUA is generating a new message.
Automatic responders can annoy Users of Mailing Lists unless they
The identity fields are relevant to a typical MUA:
4.2.2. Message Store (MS)
An MUA can employ a long-term Message Store (MS). Figure 5 depicts
an Author's MS (aMS) and a Recipient's MS (rMS). An MS can be
located on a remote server or on the same machine as the MUA.
An MS acquires messages from an MDA either proactively by a local
mechanism or even by a standardized mechanism such as SMTP(!), or
reactively by using POP or IMAP. The MUA accesses the MS either by a
local mechanism or by using POP or IMAP. Using POP for individual
message accesses, rather than for bulk transfer, is relatively rare
4.3. MHS-Level Services
4.3.1. Mail Submission Agent (MSA)
A Mail Submission Agent (MSA) accepts the message submitted by the
aMUA and enforces the policies of the hosting ADMD and the
requirements of Internet standards. An MSA represents an unusual
functional dichotomy. It represents the interests of the Author
(aMUA) during message posting, to facilitate posting success; it also
represents the interests of the MHS. In the architecture, these
responsibilities are modeled, as shown in Figure 5, by dividing the
MSA into two sub-components, aMSA and hMSA, respectively. Transfer
of responsibility for a single message, from an Author's environment
to the MHS, is called "posting". In Figure 5, it is marked as the
(S) transition, within the MSA.
The hMSA takes transit responsibility for a message that conforms to
the relevant Internet standards and to local site policies. It
rejects messages that are not in conformance. The MSA performs final
message preparation for submission and effects the transfer of
responsibility to the MHS, via the hMSA. The amount of preparation
depends upon the local implementations. Examples of aMSA tasks
include adding header fields, such as Date: and Message-ID:, and
modifying portions of the message from local notations to Internet
standards, such as expanding an address to its formal IMF
Historically, standards-based MUA/MSA message postings have used SMTP
[RFC5321]. The standard currently preferred is SUBMISSION [RFC4409].
Although SUBMISSION derives from SMTP, it uses a separate TCP port
and imposes distinct requirements, such as access authorization.
These identities are relevant to the MSA:
4.3.2. Message Transfer Agent (MTA)
A Message Transfer Agent (MTA) relays mail for one application-level
"hop". It is like a packet switch or IP router in that its job is to
make routing assessments and to move the message closer to the
Recipients. Of course, email objects are typically much larger than
the payload of a packet or datagram, and the end-to-end latencies are
typically much higher. Relaying is performed by a sequence of MTAs
until the message reaches a destination MDA. Hence, an MTA
implements both client and server MTA functionality; it does not
change addresses in the envelope or reformulate the editorial
content. A change in data form, such as to MIME Content-Transfer-
Encoding, is within the purview of an MTA, but removal or replacement
of body content is not. An MTA also adds trace information
NOTE: Within a destination ADMD, email-relaying modules can make a
variety of changes to the message, prior to delivery. In
such cases, these modules are acting as Gateways, rather than
Internet Mail uses SMTP ([RFC5321], [RFC2821], [RFC0821]) primarily
to effect point-to-point transfers between peer MTAs. Other transfer
mechanisms include Batch SMTP [RFC2442] and On-Demand Mail Relay
(ODMR) SMTP [RFC2645]. As with most network-layer mechanisms, the
Internet Mail SMTP supports a basic level of reliability, by virtue
of providing for retransmission after a temporary transfer failure.
Unlike typical packet switches (and Instant Messaging services),
Internet Mail MTAs are expected to store messages in a manner that
allows recovery across service interruptions, such as host-system
shutdown. The degree of such robustness and persistence by an MTA
can vary. The base SMTP specification provides a framework for
protocol response codes. An extensible enhancement to this framework
is defined in [RFC5248].
Although quite basic, the dominant routing mechanism for Internet
Mail is the DNS MX record [RFC1035], which specifies an MTA through
which the queried domain can be reached. This mechanism presumes a
public, or at least a common, backbone that permits any attached MTA
to connect to any other.
MTAs can perform any of these well-established roles:
Boundary MTA: An MTA that is part of an ADMD and interacts with MTAs
in other ADMDs. This is also called a Border MTA.
There can be different Boundary MTAs, according to the
direction of mail-flow.
Outbound MTA: An MTA that relays messages to other
Inbound MTA: An MTA that receives inbound SMTP
messages from MTA Relays in other
ADMDs, for example, an MTA running on
the host listed as the target of an MX
Final MTA: The MTA that transfers a message to the MDA.
These identities are relevant to the MTA:
RFC5322.Received: Set by - Relay Server
4.3.3. Mail Delivery Agent (MDA)
A transfer of responsibility from the MHS to a Recipient's
environment (mailbox) is called "delivery". In the architecture, as
depicted in Figure 5, delivery takes place within a Mail Delivery
Agent (MDA) and is shown as the (D) transition from the MHS-oriented
MDA component (hMDA) to the Recipient-oriented MDA component (rMDA).
An MDA can provide distinctive, address-based functionality, made
possible by its detailed information about the properties of the
destination address. This information might also be present
elsewhere in the Recipient's ADMD, such as at an organizational
border (Boundary) Relay. However, it is required for the MDA, if
only because the MDA is required to know where to deliver the
Like an MSA, an MDA serves two roles, as depicted in Figure 5.
Formal transfer of responsibility, called "delivery", is effected
between the two components that embody these roles and is shown as
"(D)" in Figure 5. The MHS portion (hMDA) primarily functions as a
server SMTP engine. A common additional role is to redirect the
message to an alternative address, as specified by the Recipient
addressee's preferences. The job of the Recipient portion of the MDA
(rMDA) is to perform any delivery actions that the Recipient
Transfer into the MDA is accomplished by a normal MTA transfer
mechanism. Transfer from an MDA to an MS uses an access protocol,
such as POP or IMAP.
NOTE: The term "delivery" can refer to the formal, MHS function
specified here or to the first time a message is displayed to
a Recipient. A simple, practical test for whether the MHS-
based definition applies is whether a DSN can be generated.
These identities are relevant to the MDA:
RFC5321.Return-Path: Set by - Author Originator or Mediator
The MDA records the RFC5321.MailFrom address into the
RFC5322.Received: Set by - MDA server
An MDA can record a Received: header field to indicate trace
information, including source host and receiving host domain
names and/or IP Addresses.
4.4. Transition Modes
From the origination site to the point of delivery, Internet Mail
usually follows a "push" model. That is, the Actor that holds the
message initiates transfer to the next venue, typically with SMTP
[RFC5321] or the Local Mail Transfer Protocol (LMTP) [RFC2033]. With
a "pull" model, the Actor that holds the message waits for the Actor
in the next venue to initiate a request for transfer. Standardized
mechanisms for pull-based MHS transfer are ETRN [RFC1985] and ODMR
After delivery, the Recipient's MUA (or MS) can gain access by having
the message pushed to it or by having the receiver of access pull the
message, such as by using POP [RFC1939] and IMAP [RFC3501].
4.5. Implementation and Operation
A discussion of any interesting system architecture often bogs down
when architecture and implementation are confused. An architecture
defines the conceptual functions of a service, divided into discrete
conceptual modules. An implementation of that architecture can
combine or separate architectural components, as needed for a
particular operational environment. For example, a software system
that primarily performs message relaying is an MTA, yet it might also
include MDA functionality. That same MTA system might be able to
interface with non-Internet email services and thus perform both as
an MTA and as a Gateway.
Similarly, implemented modules might be configured to form
elaborations of the architecture. An interesting example is a
distributed MS. One portion might be a remote server and another
might be local to the MUA. As discussed in [RFC1733], there are
three operational relationships among such MSs:
Online: The MS is remote, and messages are accessible only when the
MUA is attached to the MS so that the MUA will re-fetch all or
part of a message from one session to the next.
Offline: The MS is local to the User, and messages are completely
moved from any remote store, rather than (also) being retained
Disconnected: An rMS and a uMS are kept synchronized, for all or
part of their contents, while they are connected. When they are
disconnected, mail can arrive at the rMS and the User can make
changes to the uMS. The two stores are re-synchronized when they