Email Filtering

Email filtering 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.

Mail filters can be installed by the user, either as separate programs, or as part of their e-mail program (e-mail client). 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 assets.

source : http://en.wikipedia.org/wiki/Spam_filter