[sf-lug] sf-lug Digest, Vol 22, Issue 56

Rick Moen rick at linuxmafia.com
Thu Sep 27 16:27:16 PDT 2007

Quoting RBV (GoodWriter2548 at earthlink.net):

> Having said that, Rick Moen's response, below, caught my attention:
> > "you set up a separate Mail Delivery Agent (MDA) to handle that portion of
> > the task, and use your choice of best-of-breed MUA (mail user agent, i.e.,
> > mail-reading/composing program) for _that_ part of the problem."
> Wow -- what does that mean, exactly?  It sounds to this relative newbie --
> and (blush) mere *user* -- like setting up an email server on my desktop
> computer.

Nope.  It just means you're used to programs that bundle several
functions that are logically distinct and can be implemented separately.
Let me attempt a seat-of-my-pants attempt to draw you a word picture of

The most important piece of what's required to get mail around the
Internet itself are the programs (that you as a regular user are 
seldom aware of) called "Mail Transfer Agents" aka MTAs, which are SMTP
daemon processes.  These talk to each other as peers across public
networks, sending and delivering streams of SMTP mail traffic to each

Inbound mail, upon acceptance at an MTA, needs to get dumped somewhere.
Popular MTA packages include some modest implementation or other of a
facility for accepting such mail _from_ the MTA proper for local
delivery, and there are also separate LDA (local delivery agent)
packages that can be plugged in to fulfill that function and maybe add
some additional capabilities, e.g., procmail with its filtering

Mail user agents, in their simplest form, read mail that's been
deposited by an LDA (or otherwise) on the machine you're logged into, 
and hand off new outbound mail generated by the user to a local MTA for 
outbound SMTP handling.

Stub MTAs (module within some of the more-complex, fancier MUAs)
Exception to the foregoing (about MUAs handing off outbound mail):
Certain MUAs, especially novice-friendly ones like Thunderbird,
Sylpheed, Pine, etc., include integral to the program a module that
does simplified outbound SMTP, "simplified" in the sense of not being
capable of doing anything fancy, but smart enough to deliver outbound
SMTP traffic to a network-nearby full-service SMTP daemon host, e.g.,
the outbound SMTP gateway at your ISP.

You'll recall that I said that MUAs "read mail that's been
deposited by an LDA (or otherwise)".  MDAs = mail delivery agents 
are the "otherwise":  These are standalone programs to do POP3 or IMAP
fetches from MTAs on behalf of a user.  (I suppose this implies the
existence of "stub MDA" functionality inside the fancier MUAs such as
Thunderbird, pine, etc., though I'd never considered the matter in 
those terms, before).

Before you run off in terror, the above is a set of _theoretical_ 
functional categories:  Many popular MUAs bundle basic MUA mail
read/write functionality with various stub MTA + stub MDA abilities.

> On the other hand, where might a curious newbie user go to learn more about
> this exotic sounding activity?

One place:

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