Mail server clarifications

[Tony Yarusso]

Hi, I've been reading various wiki pages and whatnot, but I'm still
having a little bit of trouble wrapping my head around the differences
between various mail server software.  I've been trying to figure out
things like MTA, MDA, etc., and it also seems like some packages are a
mix of both, so I'd just really appreciate it if someone could take
the time to explain in super-simple terms how everything relates
(either by mail or IRC - I'm in #ubuntu-server as tonyyarusso with an
always-connected screen session).  Generals would be a good start, and
in terms of any specifics things I'd be interested in knowing more
about include:
postfix
exim4
dovecot
courier
procmail
cyrus
qmail

-- 
Tony Yarusso
http://tonyyarusso.com/