* levine-adsp-00 provides a superset of methods for *how* to
determine if the domain exists: the NXDOMAIN test and the "check MX &
A/AAAA" method from SMTP. It leaves it up to the implementation to
choose the algorithm that works best for it.
What I should have said was that recipients MUST do the NXDOMAIN check
and SHOULD do the more comprehensive 2821 check. It's SHOULD since it
is my impression that many MTAs do it anyway, so there's no incremental
cost.
This gives senders a baseline that they know receivers will do, and if
the receivers are more sceptical, that doesn't hurt the senders.
R's,
John
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