Here is a very good how to on setting up a mail server under ubuntu (can be modified to work under Debian Lenny as well just need to tweak some of the package versions :) )
http://flurdy.com/docs/postfix/
I’m working on tweaking my setup to get maildrop working so I can get some server side mail rules working, only down side i can see doing this is that you might see some slowness on mail being processed.