PTR должен совпадать с HELO/EHLO, в соответствии с RFC 5321
o The domain name given in the EHLO command MUST be either a primary
host name (a domain name that resolves to an address RR) or, if
the host has no name, an address literal, as described in
Section 4.1.3 and discussed further in the EHLO discussion of
Section 4.1.4.
в EHLO должен быть использован основной hostname (primary hostname).
Требовать совпадения с MX не имеет смысла, т.к. на один сервер может указывать много MX.
P.S.
в соответствии с RFC 1035
- Gateways will often have two names in separate domains, only
one of which can be primary.
т.е. primary name может быть один и по определению зоны in-addr.arpa
Host addresses are represented by domain names that have all four labels
specified. Thus data for Internet address 10.2.0.52 is located at
domain name 52.0.2.10.IN-ADDR.ARPA. The reversal, though awkward to
read, allows zones to be delegated which are exactly one network of
address space. For example, 10.IN-ADDR.ARPA can be a zone containing
data for the ARPANET, while 26.IN-ADDR.ARPA can be a separate zone for
MILNET. Address nodes are used to hold pointers to primary host names
in the normal domain space.
т.е. и HELO и PTR тоже смотрят на primary host name, который у хостов имеющих имена во многих доменах может быть только один (даже если у них несколько IP-адресов).
Средний
