> The other issue: a sending server can resolve the CNAME and rewrite the
> address on you. I saw this years ago.
>
> E.g.:
>
> foo.com <http://foo.com/> with a CNAME of “bar.com <http://bar.com/>”
> foo.com <http://foo.com/> with an MX of “some-good-mailserver.example.com
> <http://some-good-mailserver.example.com/>”
>
> Well, this is in direct violation of the DNS specification for CNAME. When
> foo.com <http://foo.com/> is a CNAME, it CANNOT have any other records, so
> the behaviour of that MX record is undefined, whether it breaks in the way
> you describe or foo.com <http://foo.com/> simply won't resolve, is too risky
> to rely on.
Exactly — I’m not saying that you *should* set a CNAME and MX under the same
FQDN, just that if you do manage to do it, stuff will break in really
unfortunate ways.
Mostly mentioning it as firepower for the original poster dealing with
“support”, in that it’s not just theoretically bad to do. It can actively break
real email delivery.
-Jeff
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