I have Exchange 2007 and 2013 and working on migration of users and groups from 2007 to 2013. I have noticed that old migrated groups work if I add an external contact to them but newly created on 2013 don't ..
When I got the group configuration it appeared that it didn't have the X400 address on the new one. after I added that address it worked ...
Is this a bug or is there a configuration I'm missing?