This document is not the place to fight spam. If you want a BCP to deprecate
the fall-back, then write one. Until all the implementations remove
fall-back to A, the correct behavior is to also fall-back to AAAA. People
(particularly the apps dev & support communities) are having a hard enough
time getting their heads around even thinking about IPv6 that making your
proposed procedural change is insane. Whatever reasons people have for not
implementing MX will not change just because they are deploying IPv6.
The current text is just fine the way it is.
I disagree. There are several good reasons for senders not falling back
to AAAA lookup. I just don't happen to think that spam filtering is one
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