If you like Wing, take a look at Prayer. It has all the features I need, and is very VERY fast. It's a stand-alone server written in C, and is GPLed.

I find it better to do all the fancy stuff like filtering "in the server" (using procmail and spamassassin, which isn't strictly in the SMTP server I know) rather than in the client.

Providing a webby interface to the user's .procmailrc (using perl/apache) shouldn't be hard, and neither should shimming a link to it into Prayer's pages.