[abiogenesis]

This is a solved problem and not very complicated. For starters the last DNS label of a hostname cannot be all numbers: https://tools.ietf.org/html/rfc1123#page-13

[garaetjjte]

But name stored in MX doesn't have to be FQDN. I can set up A record with real server IP at 1.2.3.4.example.com, then set 1.2.3.4 in MX for example.com, and it would be perfectly standard compliant record, while it probably would be misinterpreted by servers trying to be "lenient".

[teddyh]

> But name stored in MX doesn't have to be FQDN.

On the contrary, an MX record can only contain a FQDN. If you type “foo” as your target in an MX record in your domain “yourdomain.example”, what actually gets stored in the MX record is “foo.yourdomain.example.”; a perfectly normal FQDN.

[abiogenesis]

Fair point. I just tested using tcpdump and the IP address in the response doesn't in fact end with a dot.