Question about MX record

[Klinkefus, David S]

Greetings!
I had a request for any RFC, or reference material, that says 
that a zone MX record must point at a host record instead of an IP address.
We discovered a zone that had just the IP address in the MX record,
and we couldn't do a forward lookup on that IP address to trade e-mail.
Our firewall sendmail process saw the IP address as a sub-domain name
because
of the dots in the address.

Thanks in advance.

Dave K.