A whitelist is a list of e-mail addresses or domain names from which an e-mail blocking program will allow messages to be received. E-mail blocking programs, also called a spam filters, are intended to prevent most unsolicited e-mail messages (spam) from appearing in subscriber inboxes. But these programs are not perfect. Cleverly crafted spam gets through, and a few desired messages are blocked. Most Internet users can tolerate the occasional unsolicited e-mail advertisement that a spam filter misses, but are concerned by the thought that an important message might not be received. The whitelist option is a solution to the latter problem. The list can be gradually compiled over a period of time, and can be edited whenever the user wants.

Some spam filters delete suspected junk e-mail messages straightaway, but others allow the user to place them in a quarantined inbox. Periodically, the quarantined messages are observed to see if any of them are legitimate messages. This option is used by some Web-based e-mail clients in place of, or in addition to, a whitelist.