is the processing of e-mail
to organize it according to specified criteria. Most often this refers to the automatic processing of incoming messages, but the term also applies to the intervention of human intelligence in addition to anti-spam techniques
, and to outgoing emails as well as those being received.
Email filtering software inputs email. For its output, it might pass the message through unchanged for delivery to the user's mailbox, redirect the message for delivery elsewhere, or even throw the message away. Some mail filters are able to edit messages during processing.
Common uses for mail filters include removal of spam
and of computer viruses
. A less common use is to inspect outgoing e-mail
at some companies to ensure that employees comply with appropriate laws. Users might also employ a mail filter to prioritize messages, and to sort them into folders based on subject matter or other criteria.
Mail filters can be installed by the user
, either as separate programs (see links below), or as part of their e-mail program
). In e-mail programs, users can make personal, "manual" filters that then automatically filter mail according to the chosen criteria. Most e-mail programs now also have an automatic spam filtering function. 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 information technology
Mail filters have varying degrees of configurability. Sometimes they make decisions based on matching a regular expression
. Other times, keywords in the message body are used, or perhaps the e-mail 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
. Image filtering can also be used that use complex image analysis algorithms to detect skin-tones and specific body shapes normally associated with adult-images (pornographic images).