A mail filter is a piece of software which takes an input of an email message. For its output, it might pass the message through unchanged for delivery to the user's mailbox, it might redirect the message for delivery elsewhere, or it might even throw the message away. Some mail filters are able to edit messages during processing.

Users

Mail filters can be installed by the user, either as an extension of the operating system, or as part of their mail user agent. Internet service providers can also install mail filters in their mail transfer agents as a service to all of their customers. Corporations often use them to protect their employees and their IT assets.

Motivation

Common uses for mail filters include removal of spam and of computer viruses. A less common use is to inspect outgoing email at Wall Street companies to ensure that employees comply with the laws regarding insider trading. Users might also employ a mail filter to prioritize messages, and to sort them into folders based on subject matter or other criteria.

Customization

Mail filters have varying degrees of configurability. Sometimes they make decisions based on matching a regular expression. Othertimes, keywords in the message body are used, or perhaps the email address of the sender of the message. Some more advanced filters, particularly anti-spam filters, use statistical document classification techniques such as the naive Bayes classifier.

See also

• Procmail is a common mail filter for Unix systems
• Sendmail supports libmilter for mail filtering

External links

• Brightmail - An anti-spam company which uses mail filters with MTAs
• OpenProtect - An open source mail filter for Sendmail, Postfix, Exim, Qmail.