Mac Outlook 2011 through TMG

Hi,

I have a physical TMG server running SP1 on 2008R2.  I am also  running Exchange 2007 SP3. Exchange has been published through TMG to allow external domain users connected with 3G to access their mail in Outlook.

This has been successful with cell phone, IPad, and Outlook users running Windows to access their mail perfectly.
We have recently added some MacBooks to the mix and for some unknown reason those external users cannot access their mail in Outlook. They are running Mac OS X 10.6 with Outlook 2011. They can only access mail in Safari with OWA.

Are there any ideas as to the MAC discrimination?

Tks,

Guy