Thanks for mentioning POPFile, good to hear my old code mentioned on Hacker News. I wrote the original code for POPFile in Visual Basic(!) in 2001 (it was called AutoFile back then) as I wrote it initially for my own use without Microsoft Outlook. I got the idea of using Naive Bayes for my email classification after seeing the ifile extension to exmh(1). ifile dates to 1996.

I rewrote POPFile in Perl and made it work with generic POP3 servers (hence the name) and later with IMAP and more. It is still actively maintained by a group of people I've never met(!) with the last release in December: http://getpopfile.org/

(1) http://qwone.com/~jason/papers/ifile00.pdf