Quick Spam Filter (QSF) is an Open Source email classification filter, designed to be small, fast, and accurate, which works to classify incoming email as either spam or non-spam.

To recognise spam, QSF strips the text out of the email (using MIME decoding and HTML stripping) and then splits it into tokens (words, word pairs, URLs, and so on). These tokens are then looked up in a database and analysed using the Bayesian technique to see whether the email should be classified as spam or not.

The database is generated by a process of training - QSF is given two mailboxes, one containing known spam, and the other containing known non-spam, to train itself on. After training, if QSF misfiles any email, the message it got wrong can be fed back into the database, thus making QSF learn from its mistakes.