We have recently implemented greylisting as a filter layer on our mail servers to reduce the amount of spam received. It works by temporarily rejecting the first delivery attempt of an email, for a specific sender–recipient email address pair. Mail originating from legitimate mail servers (i.e., not spam servers or botnets) will successfully delivery their messages when they make their second delivery attempt 6- to 15-minutes later. Mail from spam networks usually will not make a second delivery attempt at all.
We've run a small study of the benefits of using greylisting on our servers and the initial analysis show it helps a great deal. The drawback that a message will have a delayed delivery the first time mail is sent from an unknown email address is worth the benefits. Furthermore, to lessen this disadvantage, we've whitelisted over 500 major email senders (gmail.com, yahoo.com, ebay.com…) since they can usually be trusted not to be a source of spam (and if they are, greylisting won't help anyway). Mail from these domains will not be greylisted.
If you find message delivery delays to be a problem for you, please contact us and we can adjust the whitelist for domains you receive mail from.
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