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Postfix Documentation
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Pros and cons of before-queue content filtering

  • Pro: Postfix can reject mail before the incoming SMTP mail transfer completes, so that Postfix does not have to send rejected mail back to the sender (which is usually forged anyway). Mail that is not accepted remains the responsibility of the remote SMTP client.

  • Con: The remote SMTP client expects an SMTP reply within a deadline. As the system load increases, fewer and fewer CPU cycles remain available to answer within the deadline, and eventually you either have to stop accepting mail or you have to stop filtering mail. It is for this reason that the before-queue content filter can be used only on low-traffic sites.

  • Con: Content filtering software can use lots of memory resources. In order to not run out of memory you have to reduce the number of before-filter SMTP server processes so that a burst of mail will not drive your system into the ground with too many content filter processes. This, in turn, means that SMTP clients have to wait for a long time before they receive service.

Postfix Documentation
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