Hmmmm.... Well.... Since you used "eg.", I guess you are open to use different applications then listed. CentOS comes standard with Cyrus-imapd (it's on the install CD, and you can also install it using yum). Default configuraion uses system accounts to authenticate, but it is trivial to change it to authenticate against LDAP (for example). Exim comes standard with CentOS too.
I'll do this rather than compile my own imapd+mta. The only drawback is LDAP over an SQL DB, but I guess it's worth it.
Thanks.
Morten