A whitelist is a list of email addresses or domain names from which an email blocking program will allow messages to be received.
Email blocking programs, also called spam filters , are intended to prevent most unsolicited email 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 email 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 email 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 email clients in place of, or in addition to, a whitelist.