Not sure if this has been posted here yet, but though it might be of interest. Turning your OSX box into a mailserver by replacing sendmail with Postfix. Very easy to follow guide and thorough explanations.
http://www.stepwise.com/Articles/Workbench/eart.index.html
Article by Graham Orndorff
Mac OS X Hints
http://hints.macworld.com/article.php?story=20010612102648193