the best way would be to check if Outlook uses a specific User-Agent which we could use. If you look at the access.log, the User-Agent should be logged. Can you check if it is different from a normal Internet Explorer request?
The name of Outlook User Agent Is "MSoffice 12". I have added a criteria for my rule that blocks uncatogorized URLs (and
-> Property= "Header.Request.Get(String) > Parameter "User-Agent" -> Operator="does not match" -> Parameter= Value : *MSOffice* ). This rule now does not block URL links inside email messages.
Thanks for you help.