RDNS and untangle.

Hey there.

I'm sure I'll done something wrong or mised a step somewhere along the line, but:

My untangle is in router mode.
Ext IP is 93.165.x.x / 30
It has a extra IP range: 188.180.110.x / 27

My servers are on the DMZ network, wich has ip: 192.168.31.x / 24

Port forwards is setup to:
188.180.110.a / 27 <-> 192.168.31.a /24 ports: 25, 110

When that server sends mails, sometimes the mail returns with a: RDNS fail..
Since a RDNS returns not 188.180.110.a but 93.165.x.x.

Ping does the same:
ping 188.180.110.a and I've get this:
reply from 93.165.x.x -----

Any ideas?

Kind regards, Michael.