Here's a couple of things I wrote for use with qmail.
finddups.py is a duplicate mail eliminator. Unlike the others available, it takes command line options to indicate what things to check for duplication. Those arguments are either header field names to check, or body or headers to indicate that the body or all the headers should be checked. finddups computes the md5 hash of the indicated items, and checks that against a list of recent mail, and then stores the sum in that list.
qxbiff.c is an xbiff helper written in C. The currently available ones were all scripts, and many used multiple commands to check for email. This seemed to be a bit much for such a simple task, so I rewrote it in C.