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



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