> Obviously if the domain does not exist (ie: There is no
MX or A record) attempting to send a bounce is an exercise in futility.
Checking for the actual existence of that address (ie: That there is a
mailbox that can accept the bounce message) requires the issuing of a
VEFY Command which many/most SMTP Servers will not respond to for
security/privacy reasons.
Well, you can do HELO/MAIL FROM:<>/RCPT TO:<address>/RSET, though in my
experience some servers will reject a RCPT TO (regardless of the
address) if the MAIL FROM address was empty. Yes, this means that the
bounce would never be delivered, but it _doesn't_ mean the email is invalid.
Your statement that you should be "checking the DNS name of the MAIL
FROM _before_ you agree to relay mail to a recipient" applies to the MSA
not when (as I qualified my statement) acting as a "final destination"
MTA Server since at that point you are not relaying but attempting to do
delivery.
Perhaps unfortunately, there are often many more hops than just
submission and delivery. Of course if you know than an upstream MTA has
already done the checks you can avoid doing them again, but in general
you don't know that. Ideally you should have as few MTAs as possible in
the signal path, and errors should be detected as early as possible.
I haven't tried doing any actual measurement, but my impression is that
an increasing number of large email networks are figuring this out.
Keith