For no other reason then the fact it was all laid out very well I chose
Qmail using the www.qmailrocks.org
At the end of it all you get IMAP, IMAPS, POP3, POP3S, webmail, virtual
domains, antispam, antivirus, mailing lists and remote admin via http.
The install is very straight forward, not 100% perfect, but the author
makes it clear you need to do some understanding before going live.
Many people will say Qmail is evil as it breaks the RFC, but don't
forget an RFC is only a request for comment not a standard ;) also for
just a small home / business server it gives you enough.