Happy New Year Everyone

We are experiencing an interesting problem with the post offices at one of our remote sites, where the POA's lose connection to the MTA at the main site after approximately 10hours. The other remote site and the local POA's have no problems during this time and continue to send mail. Rebooting the remote server makes no difference, as does restarting the MTA, the only fix is rebooting the MTA server.

The remote post office can recieve OK, but not send. The log file has the following entry:
01:34:50 EE7F MTP: our_domain: Unable to open connection, no response. Address = <gw-mta ip address>, Port = 7100

This seems to be some TCP/IP issue, but what? and why would it only effect one remote site and not the other?

All servers involved are OES11 and GW2012SP1

Any suggestions would be appreciated