You also have the problem of dealing with IP literals, and users running dynamic DNS which still has stale DNS data (so the response should be a 4xx code not a 5xx code, if you do something like this).
I think anyone who is running a mail server on a dynamic IP has to expect problems, and kind of deserves what they get. I say this as someone who does run one that way. ;)
As for IP literals, even though the RFC requires them a lot of MTAs don't accept them anymore.