On Tue, 8 May 2007, Dave Crocker wrote:
The header contains two very different types of information. One is between
the originator and the recipient, such as TO and Subject and Date. The other
type is part of the handling information associated with message transfer.
All of this information is for consumption by user-level agents, some
generated by the originating user agent, some by intermediate user agents
(e.g. List- and Resent- fields) and some by intermediate MTAs.
If the latter is not part of the envelope, then what is?
The STANDARD (RFC 822, RFC 1123) definition is just the MAIL and RCPT
commands and excludes all message data. The only other meaning of
envelope in Internet Mail specifications is the IMAP envelope which is
just the basic header fields that MUAs usually display, and has nothing to
do with transport or tracing. i.e. the definition in
draft-crocker-email-arch is an entirely new invention.
The envelope is for consumption by transport-level agents, and is
discarded on final delivery. (Therefore IMAP and POP don't provide access
to it because it isn't there.) This means that user agents can't see the
envelope. Any feature that depends on envelope information to work (e.g.
DSNs) must therefore be implemented by a transport agent. Any feature that
must be implemented by a user agent (e.g. MDNs) must be signalled in the
header not the envelope.
There used to be a fairly firm architectural principle that MTAs should
not have to examine the message data in order to perform their function in
the usual case. No examination is needed to add trace information.
Downgrading for 8BITMIME is an instructive example: the whole point of the
BODY= parameter on the MAIL command is so that the MTA can avoid looking
at the data in the normal cases.
The envelope/data distinction is pretty fundamental, and corresponds
fairly exactly to the user/transport layering.
Tony.
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