I think this makes sense, but I think it would make more sense if there was
a way to just specify in the SPF record for, for example, twitter.com, that
all legit senders for all subdomains are included in the highest level SPF
record.
This sort of thing has been proposed before. It turns out that
anything in the DNS that starts "all names below this node ..." is
astonishingly hard to implement.
(Yes, I know about zone cuts.)
I do agree that it would have been really nice if the SMTP fallback
from MX to A records had been deprecated and killed back in the 1980s.
I tried to get RFC 5321 not to add fallback to AAAA but the powers
that be insisted that it was already too late.
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