Greylisting is well known antispam technique. It’s idea is basing on fact, that spamming hosts (zombies) do not have time for re-transmission and are trying to send maximum amount of spam in shortest time period. This is achieved by connections to different mail servers and submission of message even without wait for server’s response. In opposition to that, legitimate mail servers presents themselves and waits for server’s response, and then beginning mail submission. If they receive temporary error code (4xx) from server, they will try to submit mail again after defined time period (e.g. 5 minutes)
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