The whole courier package includes MTA, MSA, POP3, IMAP, webmail and a partridge in a pear tree. Fortunately, it's usually packaged so you only install and enable the bits you want.

The webmail app has the quirk that it reads and writes Maildir files directly, which means you don't need IMAP to run it. However, it's not very fancy or full-featured and there are some small bugs in the stylesheet if you're using Opera. OTOH, the IMAP support is quite good.

Oh and the SMTP bit's default settings will probably need detuning, as by default it's quite anal about following the RFC.

Wade.