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What's with all these "lists"? What's the difference?

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This page details what a blacklist, whitelist and greylist is and what the difference is.

Blacklist

A blacklist is a list of clients that we deny access to a certain service. In the case of an email server, this can be a list of source email addresses, hosts and / or IP addresses that the server automatically rejects.

Whitelist

A whitelist, in relation to email servers, is a list of source hosts, email addresses and / or IP addresses that we automatically consider valid senders and thus we automatically accept all mail from these sources (ie. we don't need to do greylisting or antispam or virus checks on email from these sources -- although it's probably a good idea to always do virus checks...).

Whitelists are sometimes used to build an extremely safe antispam email box that ONLY ever accepts email sent from sources on the whitelist. This is probably a good approach for _some_ internal company addresses, or extremely private email addresses.

Greylist

A greylist is not something that is automatically maintained by a person, but it is a list of email sources (IP / hosts etc.) that have recently attempted to deliver an email to the server. Now the emails seem ok, but just to be sure we require the sending server to retry sending the message again before we move them off the greylist and to our whitelist - and actually put the email in the inbox of the receiving account (this is how greylisting works).

So one might say that a greylist is a list of senders on their way to entering our whitelist. Now the implementation can be very different on how these lists are used. Ie. the whitelist may not be permanent but reset every once in a while, and the blacklist for instance, is probably delivered by some outside service (ie. Spamhaus).