On Mon, 9 Apr 2018 09:24:23 -0500
David Jones wrote:

> I was wondering if anyone knows of an SA plugin or another method to 
> determine if the envelope-from domain has a valid MX record that is 
> listening on TCP port 25.  I don't think it would be a major scorer
> but it could be useful in meta rules.

There's NO_DNS_FOR_FROM which tests for MX or A.  I don't know if it's
still true, but historically this has, in my experience, been largely a
surrogate test for made-up domains.

Having a single test for DNS and port 25 doesn't sound like a good
idea since you can't determine how much benefit come from each of the
two parts or score them separately.   

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