Es ist nicht wirklich ein Fehler. Mir ist kein RFC bekannt, welches ausdrücklich eine eindeutige Reverse-Auflösung einfordert. Gemeinhin gilt es heute aber als Best Practise, auf einen Match zwischen A- und PTR-Record bei MXen zu prüfen.
http://www.faqs.org/rfcs/rfc1912.html 2.1 Inconsistent, Missing, or Bad Data
Every Internet-reachable host should have a name. The consequences of this are becoming more and more obvious. Many services available on the Internet will not talk to you if you aren't correctly registered in the DNS.
Make sure your PTR and A records match. For every IP address, there should be a matching PTR record in the in-addr.arpa domain. If a host is multi-homed, (more than one IP address) make sure that all IP addresses have a corresponding PTR record (not just the first one). Failure to have matching PTR and A records can cause loss of Internet services similar to not being registered in the DNS at all. Also, PTR records must point back to a valid A record, not a alias defined by a CNAME. It is highly recommended that you use some software which automates this checking, or generate your DNS data from a database which automatically creates consistent data.
Gruß, Magnus