prolaw and outlook integration with Exchange 2010

Recently, I have replaced a Exchange 2003 system with a new Exchange 2010 email server… migrated all the mailboxes over and all ok. Using BES 4.5 too, no problem..
however, Prolaw cannot work. When a user creates an appointment in Prolaw, it uses Outlook to send the email invitation out to it’s recipients.. When the recipient clicks on the email they can open it and when clicking on Accept (or deny) it pops up that the object is not accessible or no longer available.

Any suggestions? Prolaw is no help.. all they say is that it uses Outlook to send the appointments.. however using outlook (2003-2007) to create appoiments work fine..

thanks

Chad