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.  WDDX employs selective greylisting, only choosing to employ the technique if the connecting server crosses a specific threshold of suspicion.

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