A software routine that deletes incoming spam or diverts it to a "junk" mailbox (see spam folder). Also called "spam blockers," spam filters are built into a user's e-mail program. They are also built into or added onto a mail server, in which case the spam never reaches the user's mailbox. See e-mail program and mail server.

Spam filtering can be configured to trap messages based on a variety of criteria, including sender's e-mail address, specific words in the subject or message body or by the type of attachment that accompanies the message.

Blacklists and Whitelists
Address lists of habitual spammers, known as "blacklists," are continuously updated by various organizations and ISPs. Mail from blacklist addresses is rejected at the mail server. See Blacklist of Internet Advertisers.

The opposite approach is taken to ensure that bona fide mail is not automatically rejected. Users or network administrators can create a list of allowed e-mail addresses, known as a "whitelist," and the mail client or mail server will always accept mail from whitelist addresses. Mail client software typically treats the user's address book as a whitelist, presuming that mail coming from an address maintained by the user is always valid.

Analyze the Content
In order to more effectively analyze the content and not trash a real message, sophisticated spam filters use artificial intelligence (AI) techniques that look for key words and attempt to decipher their meaning in sentences (see Bayesian filtering). See spam trap, spam relay and spamdexing. See also ad blocker and popup blocker.