I'm far from one of the sharper LQ'ers on mailer programs, but I've given a few a run for their money. Currently am giving Qmail a go on my server and it seems to truly be a fine written program. It does take a bit more of a learning curve than does, say Postfix, which works out of the box, but seems to have a lower overhead (needs less resources to run, and is claimed to be more secure).
For first time mail users it's a toss up between Postfix and Qmail IMHO. I'd say steer clear of Sendmail due to it's complexity. Nothing against the app, excellent, just seems much more geared towards larger corporations. For a personal/home user or small business Postfix works great, as does Qmail. I used Postfix for a long time (still do actually for incoming mail until I can get a few things squared away to work on Qmail a bit more) and it has been great. Easy to understand documentation, easy to follow instructions, clear and easy to interpret error messages, yet works like a commercial product.
If you have absolutely no mail server experience, you can pick either one and be fine Qmail or Postfix. But if you've worked with Sendmail before, I'd have to suggest going with Postfix as it will be more familiar and easier to setup.
LifeWithQmail is one of the best documentations around IMHO. Very descriptive, walks the new user through EVERYTHING, holding your hand, describing things very well and in a human readable format (in other words, you don't have to be Linus to understand them
). And just like MC9 said:
Postfix, Sendmail and Qmail all should work fine with SpamAssassin and/or Squirrelmail. In your description above it basically tells you it uses procmail (or Sendmail/Postfix or Qmail) so you don't have to worry about which mail server (MDA) you are using. And again, as MC9 said (he's a real guru you see
) Squirrelmail depends on IMAP, and doesn't care which MDA (mail server) you use really.
Good Luck, and have fun!