[Mailman-Users] short question, probably an FAQ that I've overlooked.

[Brad Knowles]

At 8:22 PM -0500 2005-06-02, Larry Stone wrote:

>  Wow. I thought I knew how an MUA and an MTA interfaced. But what you're
>  saying is that an MUA can send the MTA a BCC header (which, of course, is in
>  the DATA section of the SMTP exchange) and rely on the MTA to strip out the
>  BCC and add the additional envelope recipients.

	Correct.  It all depends on the MUA and the MTA.

>  Considering that every MUA I use can communicate with the MTA via the SMTP
>  port, I assumed (assumed - that's dangerous) that it was sending normal SMTP
>  commands - i.e. a MAIL FROM command, one or more RCPT TO commands, and a
>  DATA command followed by the message.

	Many MUAs actually hand the message off to the MTA via the 
command-line.  They call /usr/bin/sendmail (or somesuch), and trust 
that it will do the correct things with the message.

	There's also the Submission port (587), and MTAs can be 
configured to act differently depending on whether they think that 
the incoming message is an initial submission from an MUA or if they 
think that the message coming in is being relayed to them by another 
MTA.

>                                         And I assumed (there's that word
>  again) that the MTA did nothing with the To and Cc headers other than to
>  pass them on unchanged. So is there something different going on between an
>  MUA and an MTA?

	MTAs can change the "To:" and "Cc:" headers, too.  It depends on 
whether or not the MTA in question considers itself to be the final 
destination for one or more of the recipients, or if it should be 
handing the message off to one or more other machines.

	You can get lots of variability in the interface between MUAs and 
MTAs, too.

>                   Or if not, why would an MUA not know enough about SMTP to
>  send multiple envelope recipients.

	Depends on the MUA and how much intelligence was encoded in the program.

-- 
Brad Knowles, <brad at stop.mail-abuse.org>

"Those who would give up essential Liberty, to purchase a little
temporary Safety, deserve neither Liberty nor Safety."

     -- Benjamin Franklin (1706-1790), reply of the Pennsylvania
     Assembly to the Governor, November 11, 1755

   SAGE member since 1995.  See <http://www.sage.org/> for more info.