POPFile is an abandoned free, open-source, cross-platform mail filter originally written in Perl by John Graham-Cumming and maintained by a team of volunteers.

This allows the filter to "learn" and classify mail according to the user's preferences.

In the most popular mode, it sets itself up as a proxy between the email client and the POP3 server.

An HTML based interface can be used to instruct POPFile, allowing users to correct errors in classifications and thus train the system to be sensitive to the user's specific requirements.

As an alternative to the subject-line modification (or as a supplement to it), the system can also be configured to use custom mail headers instead.