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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.
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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.
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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.
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