FWIW and in case the clarification is needed, I am in complete
agreement with Dave's analysis and comments.
Interestingly, before we formalized the role of MSAs with RFC
2476, I was using almost exactly the same terminology and
distinction about MTAs performing MUA functions that appear in
Dave's note (I believe he has been using it at least equally
long). Since we started talking about MSAs, I've gotten a bit
more sloppy because both the SMTP and MSA docs assume that they
can perform MUA functions. This discussion probably illustrates
that original distinction is worth retaining.
john
--On Tuesday, April 17, 2012 07:40 -0700 Dave Crocker
<dhc(_at_)dcrocker(_dot_)net> wrote:
On 4/16/2012 8:38 AM, John Levine wrote:
Of course, if one had a submission server that was smart
about these things, the MUA could hand the message,
including the Bcc field, to it and let it sort out envelopes
(which it needs to do anyway) and the warning message.
That strikes me as a really unfortunate idea. Submission
servers know all about envelopes and message headers. In
most cases, they don't know anything about the body of the
message beyond what they can intuit from the MIME headers.
As noted, this is a scenario that has been supported by some
posting MSAs, like
sendmail and mmdf, going back a very long time. My own view
is that this is,
nonetheless, an MUA function. That is, for this feature,
sendmail and mmdf
were/are acting as MUAs to prepare the message for formal
posting, and then they
turn into MSAs.
Note that the function is really the same as most/all MUA's
do, when processing
a message themselves for regular SMTP/Submission postings:
parse a number of
header-fields as sources for addresses to be used in the
RCPT-TO list.
This is a nice demonstration of the difference between network
architecture and
software architecture, in which different software
implementation choices can
lead to placement of network architecture functions in
different modules.
For the scenario being discussed, the "MSA" is actually part
MSA and part MUA.
If your submission server adds text to a body saying
something like "you are a blind copy recipient", in what
language is that message expressed?
That's a nice example of the basic reason to keep network
architectures as far
from user interface design as we can. (chorus: ) Presentation
and interaction
design are specialties that are not well represented in
protocol development
groups...
d/