[Mailman-Developers] Handling non-standard or obfuscated DSNs for DMARC rejects

[Stephen J. Turnbull]

It just occurred to me that (at the expense of a DNS query) many sites
can distinguish DMARC rejects pretty well by doing a DNS query for the
>From address (which I think we should be able to get, at least most of
the time).  This may not help much if you have most of your users
posting from p=reject sites (you can't easily distinguish an
obfuscated DMARC reject from a broken recipient address), but could
save some of your users trouble if you have enough non-reject posters,
or those users have standard DSNs that specify a DMARC reject.

Of course you'd want to cache the result, I guess -- and actually, it
would already available from when you accepted the post in the first
place (if you have a non-null dmarc_moderation_action).