I have been using Qmail. http://www.qmailrocks.org/ This site has everything you will need.
Maybe a little too general and/or enthusiastic
;->
Qmailrocks has not been maintained for about 1 to 2 years
If you use qmailrocks.org, you must also consult http://qmail.jms1.net and several other places or you will waste a lot of your time
This is not a two second mail server although once fully understood and properly implemented it is one of several good choices.
You should be a solid linux or unicie administrator before you even consider doing it.
- rh