How to set up a mail server on a GNU / Linux system
While in college, my housemates had a Linux mail server configured with individualized spam filtering; every user had an IMAP "spam" folder and a "ham" folder; they could move false negatives from their inbox to the spam folder, move false positives from spam to ham, and a nightly job would run and generate custom statistics for each user.  It was remarkably slick, and for years I've been trying to figure out what that setup was.  Does anybody have any ideas, or actual links to similar tutorials?