BIND 9 - Reverse lookups made by SMTP servers fail

[SZABO Stefan]

Hello,
 
Our 2 Bind servers encounter problems to answer reverse requests made by
SMTP gateways.
 
The 2 dns servers are not in the same version (but both are 9.2.x).
Reverse lookups fail on MX records, sometimes. In addition, the problem
is random, and hard to reproduce on demand.
 
SMTP gateways then consider that DNS did not anwser, as if the DNS
did not send any answer to the reverse lookup request.
 
Root Servers file is up to date.
 
In Event Viewer , RCODE SERVFAIL and REFUSED messages can be found, but
I'm not
sure that those event are related to failed reverse lookups.
 
Stefan - Network Engineer.