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agent
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... ASCII characters before invoking
a local mail UA (User Agent, a program with which human users send
and receive mail). Examples of such encodings ...
... version number to
declare a message to be conformant with this specification and
allows mail processing agents to distinguish between such
messages and those generated by older or non-conformant software,
which is presumed to lack such a field.
...
... compatible with RFC 822std11(-> 2822prop), there are still circumstances in which it
might be desirable for a mail-processing agent to know whether a
message was composed with the new standard in mind.
...
... Content-Type field is to describe the data
contained in the body fully enough that the receiving user agent can
pick an appropriate agent or mechanism to present the data to the
...
... receiving user agent can
pick an appropriate agent or mechanism to present the data to the
user, or otherwise deal with the data in an appropriate manner.
...
... Content-Type of "image/xyz" is enough to tell
a user agent that the data is an image, even if the user agent has no
...
... a user agent that the data is an image, even if the user agent has no
knowledge of the specific image format "xyz". Such information can
...
... starting with "X-") may be
defined bilaterally between two cooperating
agents without outside registration or
standardization.
...
... Version header field, a receiving User Agent can also assume that
plain US-ASCII text was the sender ...
... with little potential benefit. Their use is allowed only as the
result of an agreement between cooperating user agents.
If a Content-Transfer-Encoding ...
... order to be properly viewed. Nested encodings add considerable
complexity to user agents: aside from the obvious efficiency
problems with such multiple encodings, they can obscure the basic
...
... gateways, but this seems less
of a problem than the effect of nested encodings on user agents.
NOTE ON ...
... rule is necessary because some MTAs (Message Transport Agents,
programs which transport messages from one user to another, or
...
... deleted, as it will necessarily have been added by intermediate
transport agents.
Rule #4 (Line Breaks ...
... This provides a mechanism with which long lines are encoded in
such a way as to be restored by the user agent. The 76 character
limit does not count the trailing CRLF, but counts all other
...
...
In constructing a high-level user agent, it may be desirable to allow
one body to make reference to another. Accordingly, bodies may be
labeled using the "Content-ID ...
... practice. In order to eliminate such ambiguity and variations in the
future, it is strongly recommended that new user agents explicitly
specify a character set via the Content-Type ...
... encapsulation boundary MUST NOT appear inside any of
the encapsulated parts. Thus, it is crucial that the composing agent
be able to choose and specify the unique boundary that will separate
the parts.
...
... required syntax for the multipart type. This requirement ensures
that all conformant user agents will at least be able to recognize
and separate the parts of any multipart entity, even of an
...
...
Mail gateways, relays, and other mail handling agents are commonly
known to alter the top-level header ...
... encapsulation boundaries must not appear in the body
parts being encapsulated, a user agent must exercise care to
choose a unique boundary. The boundary in the example above could
have been the result of an algorithm ...
... Alternate algorithms might result in more 'readable' boundaries
for a recipient with an old user agent, but would require more
attention to the possibility that the boundary might appear in the
encapsulated ...
... capabilities of their system.
In general, user agents that compose multipart/alternative entities
must place the body parts in increasing order of preference, that is,
with the preferred format last. For fancy text, the sending user
agent ...
... user agents that compose multipart/alternative entities
must place the body parts in increasing order of preference, that is,
with the preferred format last. For fancy text, the sending user
agent should put the plainest format first and the richest format
last. Receiving user agents ...
... user
agent should put the plainest format first and the richest format
last. Receiving user agents should pick and display the last format
they are capable of displaying. In the case where one of the
alternatives is itself of type "multipart" and contains unrecognized
...
... they are capable of displaying. In the case where one of the
alternatives is itself of type "multipart" and contains unrecognized
sub-parts, the user agent may choose either to show that alternative,
an earlier alternative, or both.
...
... more important in this case.
It may be the case that some user agents, if they can recognize more
than one of the formats, will prefer to offer the user the choice of
...
... simultaneously on hardware and software that are capable of doing so.
However, composing agents should be aware that many mail readers will
lack this capability and will show the parts serially in any event.
...
...
Mail gateways, relays, and other mail handling agents are commonly
known to alter the top-level header ...
... objects to be delivered as several separate pieces of mail and
automatically reassembled by the receiving user agent. (The concept
is similar to IP fragmentation/reassembly ...
... Internet
Protocols.) This mechanism can be used when intermediate transport
agents limit the size of individual messages that can be sent.
Content-Type "message/partial" thus indicates that the body contains
...
... prohibited for entities of type message/partial.
It should be noted that, because some message transfer agents may
choose to automatically fragment large messages, and because such
...
... choose to automatically fragment large messages, and because such
agents may use different fragmentation thresholds, it is possible
...
... For example, a meeting scheduler might define a standard
representation for information about proposed meeting dates. An
intelligent user agent would use this information to conduct a dialog
with the user, and might then send further mail based on that dialog.
More generally, there have been several "active ...
... number of useful Content-Types are defined for general use by
consenting user agents, notably message/partial, and
message/external-body.
...
... image, or video, should also be treated in this way.
A user agent that meets the above conditions is said to be MIME-
conformant. The meaning of this phrase is that it is assumed to be
...
... any known systems that are conformant with RFC 821std10(-> 2821prop) and RFC 822std11(-> 2822prop). User
agents that are MIME-conformant have the additional guarantee that
the user will not be shown data that were never intended to be viewed
...
... encoding used for data may change
as part of normal gateway or user agent operation. In particular,
conversion from base64 to quoted-printable ...
... (5) Trailing "white space" characters (SPACE, TAB (HT)) on a line
may be discarded by some transport agents, while other transport
agents ...
... agents, while other transport
agents may pad lines with these characters so that all lines in a
mail file are of equal length. The persistence of trailing white
space, therefore, must not be relied on.
...
... characters.
(7) Some mail transport agents will corrupt data that includes
certain literal strings. In particular, a period (".") alone on a
...
... "From " (the fifth character is a SPACE) are commonly corrupted as
well. A careful composition agent can prevent these corruptions
by encoding the data (e.g., in the quoted-printable ...
... security problems could result. Authors of mail-reading
agents are cautioned against giving their systems the power
to execute mail-based application data without carefully
...
... Borenstein, N., "A User Agent Configuration Mechanism for Multimedia Mail Format Information", RFC 1343, Bellcore, June 1992. ...