Your only option is to turn off BOFHCHECKDNS, which turns off ALL dns checking, letting in all sorts of crap.
It's much better to tell the sender to fix their dns.
Okay, I'll do that. In this case, the DNS hosting firm is local, so I should be able to do this.
I'm curious, though -- and I'm sure this has been hashed out before -- what is the problem with having a raw IP address in an MX record? I understand it's a technical violation of RFC 1035, but I've never been able to fathom just what vulnerability it opens up or other problem it causes. My ignorance here makes it hard to convince other firms just why they should "fix" their DNS. The response typically is "Okay, it's a technical violation. So what?"
MX records contain hostnames, not IP addresses. Normal processing of MX records will result in the malformed MX record getting ignored (since the A lookup on the hostname will fail).
So, with none the wiser, the MX record will be ignored. This may not be noticable right away, and everyone will carry on, forging ahead for some time, before anyone realizes that this MX ain't getting much mail.
This is a special-case testing for a common misconfiguration, and explicitly rejecting it in a visible way, so that it may be fixed.
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