Am 19.02.2018 um 13:12 schrieb Ralph Krämer:
I am sure this is mentioned somewhere in one of the RFCs - but I currently have no time to look this up.
rfc1912 references in "CNAME records" section
Don't use CNAMEs in combination with RRs which point to other names like MX, CNAME, PTR and NS. (PTR is an exception if you want to implement classless in-addr delegation.)
to rfc974, which sez in "Minor Special Issues" section
Note that the algorithm to delete irrelevant RRs breaks if LOCAL has a alias and the alias is listed in the MX records for REMOTE. (E.g. REMOTE has an MX of ALIAS, where ALIAS has a CNAME of LOCAL). This can be avoided if aliases are never used in the data section of MX RRs
I know of several big esp that reject if rcpt mx is resolved by a cname-rr
Cheers
tobi