I ha to temporarily disable this option because of bad DNS config from
one of our clients.
Now the client argue that this setting is not in any RFC so it does not
want to fix the MX - PTR issue !
Is there a RFC or a good document that I can you to prove my point :)
Section 3.3.9 defines the "exchange" value as a <domain-name>. It
defines <domain-name> as a series of labels, and not as IP addresses.
The RFC notes that MX records cause type A additional processing, and
does not specify that type CNAME additional processing is allowed.
RFC 974 may further recommend that aliases are never used in the data
section of MX RRs.
In the end, though, your client's attitude won't get mail delivered.
Using names that resolve to A records will. One of those things is
useful. Tell him to suck it up and cooperate a little.