From POP3 To IMAP-What Solutions Are There?

Rhyder asks: "Users were once-happy with a POP3 client and their single desktop or laptop. As the productive portion of the user-base (20% do 80% of the work) moves up in knowledge, implementation and expectations, they simply realize that there should exist something better than a POP3 client. The upper 10% further realize that there must exist an open standard, something beyond Lotus Notes and MS Exchange. Should I implement a GNU server or a commercial server? I refrain from mentioning any packages here, as I do not want to bias a selection. But really, what is the best approach for an IMAP4 server with the posibility of adding on Radius Authentication, Web-based e-mail, possibly a directory service as well? Should a Sendmail core be implemented, or at least used as an MTA? Oh yeah, which OS - Solaris, Linux, BSD, or something else?"