Dear Robert.
I have decided to give SME a go. It provides Qmail on Centos 4.7, with Centos 5.2 in beta.
I chose SME because I also have to replace an NT server here as well, so it makes a good fit.
I have a test system working and building the mailserver replacement system now. Then I will build the NT server replacement.
You can also take a look at the very promising SOGo project:
http://www.scalableogo.org/english.html
It makes use of standard components (CardDAV, WebDAV...) and there are CentOS RPM packages available, too.
Best Regards Marcus