You are missing a lot, including the fact mentioned several times in this discussion that the data portion of an MX resource record simply is not an IP address.

You are missing a lot, including the fact mentioned several times in this discussion that the data portion of an MX resource record simply is not an IP address. The data structure is a wire-format domain name (plus an integer). Even in BIND's "zone" file format, a human-format IP address isn't a fully qualified human-format domain name. But yes, the DNS RFCs are not as good as they could have been. However, you have reached that conclusion by the wrong route. (-: