O'Reilly Network: Using Qpsmtpd:

In 2001, Ask Bjørn Hansen needed to improve the spam detection on perl.org's mail server. He decided the option of patching the existing Qmail installation was too painful. Instead he turned to the code in Jim Winstead's Colobus NNTP server for inspiration to re-create qmail-smtpd in Perl. The first version he hacked together comprised just 300 lines of code.