Exchange 2010 and BIG IP LTM - Out of Office

I have a pair of F5's up and running in our lab and Exchange 2010 running as per our live environment.

 

Testing is going well with email flowing through the F5's and failover testing working as expected.

 

However now the Outlook clients cannot set Out of Office. If a client clicks on the Out of Office in Outlook we get an error stating the Server is down or unavailable.

 

Why would this be if the mailbox is working fine?

 

All the settings in the iAPP seem Ok but I may have missed something I suppose as I am new to F5's.

 

Any help appreciated.

 

Rob