John Levine wrote:
* 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.
In the interest of accuracy:
I have been probing some email receive-side folk, about their call-back or
verification steps like this that they conduct. Generally, the range of tests
is of these types.
Unfortunately, it is rarely based on the rfc2822.From field. It is,
instead, typically based on rfc2821.mailfrom.
So we need to be careful about assuming that any of these tests are likely to
be "free". In fact, one bit of feedback I got was explicit about these
additional tests as costing too much. They had tried and found they added too
much delay.
FWIW.
d/
--
Dave Crocker
Brandenburg InternetWorking
bbiw.net
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