Whitelist

A whitelist is a list or register of entities that are being provided a particular privilege, service, mobility, access or recognition. Entities on the list will be accepted, approved and/or recognized. Whitelisting is the reverse of blacklisting, the practice of identifying entities that are denied, unrecognised, or ostracised.

Spam filters that come with email clients have both whitelists and blacklists of senders and keywords to look for in emails. If a spam filter keeps a whitelist, mail from the listed email addresses, domains, and/or IP address will always be allowed.

Typically, a user has to manually add the sender or the sender’s address to a list of “approved” or “safe senders” within his or her email client for an address to be whitelisted, which is a different process in various email clients.

Some Internet service providers have whitelists that they use to filter email to be delivered to their customers.

If a whitelist is exclusive, only email from entities on the whitelist will get through. If it is not exclusive, it prevents email from being deleted or sent to the junk mail folder by the spam filter. Usually, only end-users would set a spam filter to delete all emails from sources not on the whitelist, not Internet service providers or email services.

Using whitelists and blacklists can assist in blocking unwanted messages and allowing wanted messages to get through, but they are not perfect. Email whitelists are used to reduce the incidence of false positives, often based on the assumption that most legitimate mail will be from a relatively small and fixed set of senders. To block a high percentage of spam, email filters have to be continuously updated as email spam senders create new email addresses to email from or new keywords to use in their email which allows the email to slip through.

Amazon.com uses whitelists to limit access to its Kindle e-reader devices. Besides Amazon itself, only e-mail addresses whitelisted by the device's registered owner can send content ("personal documents") to that device.

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Wikipedia