Grey-listing tells newly seen emailers to "retry". They are not whitelisted until they come back with the same request, 300 or more seconds later. Spam delivery programs do not usually retry as it is expensive in CPU and disk resources for them to do so when they are delivering thousands of emails a minute. This should dramatically reduce the amount of spam.
Most spam is sent from infected home PCs in private homes. The spambots running on a PC do not have any logic for resending a failed message. These applications appear to adopt the "fire-and-forget" methodology. A decent mail server however, will try to deliver a message for at least three days.
Mail is transfered between mail servers using the protocol SMTP, defined in RFC2821. During delivery the receiving server will respond with three-digit codes:
* 2xx means OK
* 4xx notifies that a temporary error has occurred. The sending server should retry delivering the message.
* 5xx is a permanent rejection, ie "user unknown". The sending server should give up on the message, reporting the failed delivery to the sender.
While a real mail server will retry after the reception of a 421-message, spammers will not!
The mail server will reject a message the first time it is seen. After 3 minutes a message from the same sender and host to a local user will be accepted. After this reception all further mails will be accepted with no further delay.
There is more in-depth info at:
»projects.puremagic.com/greylisti···per.html
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by KeysCapt 
last modified: 2005-12-19 21:45:38 |
