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Exactly, which is why validating the MX is better than merely testing for NXDOMAIN. If you try delivering to something with implicit MX it is far more likely to fail than something with proper MX records. It's also more likely to fail because the A record doesn't point to a host running SMTP, so your provider will be forced to queue it and keep retrying. SMTP servers pointed to by stale MX records are much more likely to return permanent failure right away.

Nobody who actually cares about reliably receiving mail should depend on implicit MX. Of all the things mail administrators have to do these days, like reverse lookup, SPF, and DKIM, is it reasonable to care about anyone who can't be bothered with MX records?

I'd gladly reject such domains if doing so improved overall mail delivery rate to popular domains. And unlike 99% of situations where an email fails to show up, the sending web site can instantly report the reason for failure.

You bring up a very good point that I hadn't really thought of before ("... is it reasonable to care about anyone who can't be bothered with MX records?") but that has caused me to stop and think.

I manage e-mail systems with thousands of users and a fair number of domains as well. In most cases, I also manage the authoritative DNS servers for these domains so I make sure that all the appropriate records (MX, TXTs for SPF, DKIM, etc.) are set up properly.

You're right, though. If example.com hasn't bothered setting up an MX record for example.com -- even if mail is hosted on the same machine as identified in the A RR -- they probably aren't worth worrying about. It seems reasonable to conclude that they really aren't too concerned about being able to receive mail.

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