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.
Common uses for mail fitlers 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.
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.