Green Mail Filter is a server platform to build a wide variety of mail filters. When installed for a single user, it comes with a few sample sets of rules implementing the most widely used types of filtering - bayesian, blacklist, and challenge-response whitelist. Each incoming message is run through a set of rules and at the end is dispatched to either "mail", "spam" or other mailbox. The user then connects to the built-in POP3 server and reads the filtered mail.

For a developer, or a user with programming knowledge, Green offers a way to build mail filters. The rules the user can edit are based on regular expressions and Python scripts.