I maintain my own mail-server (posfix) on a linux server (fedora core 4).
I have come to hate spam and the waste of resources which comes with it.
I think about 80 % of all mail to me is spam.
But during this weekend I have reconfigure my postfix-server to get rid of all SPAM.
I found this really good article about Filtering spam with Postfix.
It was really easy to filtering spam with postfix; now 90 % of SPAM Email has now disappeared from my Email server.
I haven't yet integrated SpamAssassin or Dspam with postfix, after I have done that more SPAM will be removed.
But now it is possible again to using my Nokia phone to read mail!
There is also an Open Source product called Zimbra Collaboration Suite (ZCS), Open Source Edition that could be interesting (I haven’t tried it my self). The idea behind Zimbra is provide a do-all 'black box' that Just Works. All needed software is included. Read more about it here.
Using PostFix To Reject Spam
How to install Postfix, Amavisd-new, SpamAssassin, Pyzor, Razor, DCC, and ClamAV on Fedora Core 4 - v2.1.8
How to get Dspam, Postfix, and Procmail to play well together
Wiki, Integrating DSPAM with Postfix
Nine ways to combat spam