Qmail is a free mail server written from scratch by Dan J. Bernstein, a professor at the University of Illinois. It's small, very fast and full-featured. It was written to be completely secure, which is something most software designers will not say, but there is still an unclaimed cash price for discovering a security hole. (Note: This must be a security hole in the Qmail package, it does not apply to poorly configured and patched servers)

The following link is a report about setting up Qmail to stop or mark spam