Will a mail relay be blacklisted if it accepts a mail relay request then later sends NDR?

FuzzyWuzzyBear asked:

Got a question about blacklisting behaviour. Internet MX records for “mydomain.com” point to my anti-spam gateways. The anti-spam gateways then relay the email to my mail server. If someone sends an email to ,say, nosuchperson@mydomain.com the email is first accepted by the anti-spam gateway, but it will then be rejected by the downstream mail server because the recipient does not exist.

Now, my colleague says that he’s seen this behaviour result in a mail relay getting blacklisted (he gave AOL as an example). I have not seen this myself, but I’m not sure if its because I’ve been lucky or if its because he’s wrong. Can anybody clarify this?

My answer:

Maybe. This is called backscatter, and some services do take it into account in computing IP reputation, and some blacklists operate exclusively based on backscatter.

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