I have run several Slackware based mailservers in the past. I always used Sendmail as the MTA and
Cyrus-IMAP for the storage and IMAP/POP support. This has always worked well and is rather straight-forward to setup, but currently I have no companies running a Slack mailserver that I support.
At home, I run Cyrus-IMAP as well, and Sendmail which is specially configured so that I could plug in
Amavisd-new which is now scanning every email that passes for viruses (using
ClamAV ) and for SPAM (using
SpamAssassin ).
I have never written down a good tutorial on how to set this up, it is not completely trivial. Maybe I should do that some time... but there are a lot of pointers on the Internet (use Google!) to configure all that. Especially the Amavisd-new help pages are great if you want to know how to integrate virus- and spam scanning into sendmail or postfix.
Slackware of course does not come with a Postfix package, but Slackware 10.2
does have SMTP AUTH support in it's Sendmail package.
Eric