We have a Hybrid exchange solution with Exchange 2007 and Office 365 with on-premise Exchange 2013 hybrid server.
Steadily clients have been getting prompted for username and password when not onsite. At first it was just a few now it is growing.
I have found some articles about Calendar.
All the clients are still on the Exchange 2007 server as we have not migrated anyone and some will stay on the Exchange 2007 box due to Call Manager.
Can someone point me in the right direction, it is obviously a authentication issue and they are coming through a Microsoft TMG 2010 firewall.
The outlook clients are a mix of Office 2010 and Office 2013 and both show same symptoms.