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When configuring a mail server, what is the best practice for rejecting an inbound internet email with an executable type I'd like to disallow? Easiest option is an NDR/DSN, but I'd think that could put one at risk to create backscatter.

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serverfault.com/q/329162 –  quanta Nov 25 '11 at 19:24

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When the mail server is sendmail, I install MIMEDefang use it to enforce attachment policy.

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...which is especially useful since it is a so-called "milter" and allows for real-time filtering. This way, the mail server simply would send a reject response upon reception of the DATA section - NDRs would be someone else's problem then. Sendmail is not the only mailserver to support this, Postfix' before-queue content filters will do the same. –  the-wabbit Nov 25 '11 at 21:17
    
Thanks guys. I read that many MTAs won't honor a reject code like 550 during the DATA section of the conversation, which is concerning. –  Ode Nov 29 '11 at 15:55

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