It's mostly code snippets from the mail server project I started years and years ago. (Watch out for more on qpsmtpd later this month or next...)
It's a nice cycle. The Qpsmtpd plugin system is based, at least in spirit and some of the conventions, on the Apache module system and since 2002 I had a make a mod_perl smtp handler on the TODO. Last year Will Whittaker proved the mod_perl API was ready by making Apache::SMTP (does that still work? Didn't the mod_perl API change?) and Matt Sergeant wrote us Apache::Qpsmtpd which is now used at apache.org for a couple of million incoming emails a day.