Here are my 2 cents
I have been using Qmailtoaster (qmailtoaster.com) for a while on Centos.
Basically download the rpms and rebuild them.. Nice web based admin tools too. There are some nice user contributed installers for it also.
Nate has a nice script that basically auto installs everything. http://www.askdavis.com/qmailtoaster/
I just finished a script that installs Clamav 0.8x+Spamassassin 2.64 +Squirrelmail+SASQL+QS 1.24st as well as a few other customization.
http://mybashadmin.sourceforge.net/ToasterAddon.sh/
with these 2 scripts I can install a mail server in basically in the time it takes to download the files