I just upgraded my environment from Exchange 2007 SP3 to Exchange 2013 CU7 i coexist mode. We have 2 CAS servers (2007 and 2013) and 2 Mailbox server (2007 and 2013). Everything work – OWA, autodiscover. But the only way I can get the client Outlook to work is if I manually add “DS Server” in the registry and point to the DC. I cannot undestand why client computer can’t see the GC? Some strange thing also – there is not problem for Outlook clients to connect in virtual workstations or Citrix clients (the same user) !