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Re: Processing after the end of DATA

2010-08-10 13:38:40

Carl S. Gutekunst wrote:

Rejecting anything that you can at the SMTP level, rather than generating a bounce, strikes me as a sufficiently Good Thing that it outweighs any remaining benefits from the "MUST minimize" rule. Minimizing bounces seems more valuable than minimizing SMTP connection time, primarily because the bounces are more likely to waste an actual human being's time at some later point. -- Nathaniel

Agree with this, but I still don't run my spam filters before the final ACCEPT. As long as there is a possibility that the filter will take too long, it isn't worth the confusion resulting from a timeout.

Almost all of the commercial enterprise-grade filters I know do *all* of their filtering before accept: spam, virus, content policies, etc. That's because of the desire to emit a clear rejection, and backscatter is a far more serious problem than duplicates. The exceptions (those that do filtering after the 250 OK) don't generate DSNs; they deliver to a spam folder or other quarantine.


My system is too small to draw any conclusions, but my impression is that we are overly-concerned about a very small fraction of the mailflow - those few messages that are legit, but get a false reject from a statistical filter. We really don't care about the proper SMTP response if it is spam. The only advantage I see in running the spam filter before ACCEPT is a few less messages at the bottom of my spam bucket. I wish I had some numbers from a larger system.

Would be nice if SMTP had a 'conditional ACCEPT': "Sorry, your transmitter ID '' cannot be verified, Your message has been routed to the recipient's quarantine." Imagine the effect that would have on all the lame excuses for an invalid or hard-to-verify HELO name.

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