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Greylisting is an extremely valuable and effective mechanism for eliminating spam. When a sender tries to send a message, and they have not contacted the intended recipient before, they are initially refused with a temporary error. This will cause the message to be requeued by the senders mail server. Within a few minutes, the senders server will try again and this time the message will be allowed to pass. Once the sender tries a second time to send a message, the senders information will be added to a database and  will not need to pass greylisting again as they contact the user on a regular basis. If there is no contact for a specified number of days, or the senders address changes, the record will be purged and the sender must "re-authorize".


One of the reasons that greylisting is so effective is that a high percentage of spam now comes from computers infected with "Zombie" agents or "SpamBots". These types of hidden programs allow control of users computers without their knowledge or consent. Spam messages are sent out in the background so the user never knows they are being sent. These types of programs cannot handle greylisting, so we can basically completely eliminate them as a spam source.


For a more technical discussion of greylisting theory, click here. Our implementation differs from other implementations in that if it is used with MXRate, a high number of trusted sources will not be subject to greylist verification. It also differs in that you can set greylist triggers that force greylisting only when suspicious activity is suspected.



When Alligate is first installed and put into production, some end users that use email constantly throughout the day may complain about message delays. This is because at first, your database of greylist approved senders will be empty. This is completely normal and the end user complaints usually disappear after 3 or 4 days. It is sometimes wise to inform end users in advance that there some messages may be delayed initially for anywhere from a few minutes a up to an hour until the greylisting database gets populated with normal message senders. Most users never even notice the delays.