Oh, yeah, I also forgot to mention that sending them email from Hotmail or Yahoo works fine.
That's because many mailservers are not strictly RFC compliant, and unintentionally send to MX records that have IPs instead of hostnames, due to the way that most DNS resolvers work (if you ask for the IP for a hostname, but instead give it an IP, it gives you back the IP rather than complaining that the hostname is invalid).
I attribute this to Imail being less forgiving of DNS errors than the Yahoo or Hotmail servers. Is that correct?
Correct (and I'm guessing IMail is less forgiving intentionally).
For instance, as John showed, querying directly for the MX record seems to work, but not when you try to follow the logical path from the root servers you can't get there. How does that work?
I didn't check his results -- but no matter how he did it, he does not get a valid MX record back. An MX record that has an IP address in it is invalid (it's OK if the DNS server returns an MX record with a hostname in it *and* another record that gives the IP of the hostname, but it is illegal for the MX record to have an IP in it).
How can a direct query return the MX records when there are no NS records for the domain?
If there are no NS records for the domain, you can't do the lookup -- either directly or indirectly -- unless you have unpublished information (which of course you can't look up).
-Scott
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