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Graylisting Change language:    

With the SMTP-protocol a SMTP-server knows about three things:

  • the IP of the sender
  • the email sender
  • the email recipient

    If the SMTP-server hasn't yet received any mails with this tripple combination, a SMTP-server with graylisting answers with a tempfail error. This means that the sender should try again later to submit the mail. After some defined amount of time, a message with the same adress/ip tripple will be accepted.

    Typical spammer software doesn't try to send spams twice to the same SMTP-server since it wastes too much time. Graylisting successfully filters those mails. With graylisting you can defend spam very effective and reduce the amount of spam up to 90%. In opposite to heurisic Anti-Spam measures no email gets lost. There is the possibility of a delay in the delivery of email if graylisting is used.

    Our own implemention, (selective graylisting) tries to circumvent this delay. A big whitelist keeps the well known relays out from graylist processing. Additionally we detect the ratio of spam/ham the sending IP has sent us in history. We use graylisting only on those relays where the amount of spam is a clear sign for abuse. All other SMTP connections are accepted without delay. With this method it's not possible to reduce the amount of spam like it would be with generic graylisting, but we managed to reduce the amount of spam at least up to 30%.

    More about graylisting on Wikipedia.







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