To rule our any quirks in Office 2011, I installled an Exchange-based mail account in the MAIL app. The results were identical to Office 2011,.i.e. it works without the proxy server but does not connect to the Exchange Server with the proxy (PAC file) involved.
Any help would be appreciated.
I was having a similar problem using Outlook 2011 running under Mountain Lion (10.8.2) on an MacBook Pro. When at work, behind our proxy server, I could not get Outlook to connect to a client's public Exchange server, but this worked fine without changing any settings when I was connected to our DMZ network or at home (no proxy server in these cases). My MacBook was configured to use "Auto Proxy Configuration", and Safari worked fine in all three locations (as did Apple mail)
Today, the Microsoft Office auto-updater downloaded an update, and since it was installed, the problem has been fixed and I've been able to connect to our client's external Exchange server event when behind our local proxy server (Outlook now reports that the latest installed update is 14.2.5)