[sf-lug] interesting artifact re system mail configuration

Rick Moen rick at linuxmafia.com
Mon Feb 2 14:09:31 PST 2009

Quoting Asheesh Laroia (asheesh at asheesh.org):

> I think that our friend Rick has a page about this somewhere. (-:

Actually, I'd not hear the term "Mail Submission Agent" before -- but
apparently the term's used.

Looks like it's a sort of front-end daemon that receives outgoing mail
and authenticates the user to be authorised to originate such mail
(e.g., using one of the variants of SMTP AUTH, accepting outbound mail
on port 587/tcp).  Plus, apparently sometimes it rewrites the outgoing
mail's headers to clean them up, quality-checks the mail to try to make
sure the originating machine's not a zombified Windows box cranking out
spam or malware, and otherwise checks it out and tidies it.

So, no, I don't have any pages detailing that, but it sounds like a
reasonable invention.

The super-traditional, very old-school model for mail flow is as
follows:  Mail arrives at a receiving MTA (examples:  sendmail, exim,
Postfix, QMail, Courier-MTA), which then hands it off to a local
delivery agent (LDA; example: procmail) that deposits it in the desired
format somewhere on the MTA box's local filesystem (a spool).  At a
later time, a user telnets/SSHes in and runs a console-type client mail
program (a mail user agent = MUA), which reads the spool and permits the
addressed user to compose replies and new mails.  Those outbound mails
are then handed off to the MTA for outbound processing.

A slightly less ancient elaboration on that model introduced Mail
Delivery Agents (MDAs), daemons that offer POP3 (Post Office Protocol v.
3) or IMAP4 (Internet Message Access Protocol) remote retrieval of
messages from the local MTA spool, for the benefit of MUAs running on
users' client machines.  In those cases, the MUA either has to hand off
outbound mail to MTA running on that user machine, or has to have a
small amount of SMTP code built in, sufficient to relay it outbound to
a real MTA willing to deliver the MUA's outbound mail.

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