I just took some time now to go searching for webmail scripts that we could try to plug into w3t:
NOCC Webmail: PHP. The most convincing one to me right now... can use sendmail or smtp, checks imap and pop3, multiple languages, GPL license, changelog reflects its development is alive, looks like it would plug in fine, multiple platforms. One thing though, it does not need a database, so I am not sure if that would make it easier or harder. The thing that you should remember is that you dont want to host the email, just provide a way to read/write, so you dont really need to write much into a database anyway...
Popper: PHP, MySQL. Another great one, though it almost looks too much like Outlook Express (so not much like w3t). This one does use a database, can also send by sendmail, multiple languages, and some others...
RYMO: PHP. Looks like a very simple but fine script.
Mateo Byler
CruceDeCaminos.com