Lately I've been really interested in doing some very unique things with email most of which rely on a SMTP and POP or IMAP server that gives the administrator an API to create arbitrary methods for email storage, notifications, or delivery. What I'm looking for would be analogous to mod_php and apache where apache handles the delivery protocol and php handles the content creation and storage. I've considered making my own, as those three protocols are quite simple, but I'm always nervous about putting my code public facing especially when it's at that low of a level.

So are there any email servers that allow for this much arbitrary control over email delivery, fetching, and receiving.