Continuous password prompt from Mail and Outlook 2011

We've recently started to deploy MBP15s at our office and some of the users are using Apple Mail configured to our corporate exchange server. A couple have reported an issue of being prompted continuously to enter their network password. I've also just installed Offce for Mac 2011, setup Outlook and I've been prompted more times than I can count, to enter my password.

Is there any reason for this to continually happen?