Sounds like the Kerberos auth issue described by Ross in this post http://blogs.technet.com/b/exchange/archive/2015/02/20/exchange-2013-and-exchange-2010-coexistence-with-kerberos-authentication.aspx
From: [email protected] [mailto:[email protected]] On Behalf Of Paul Cookman Sent: Thursday, October 22, 2015 8:40 AM To: '[email protected]' <[email protected]> Subject: [Exchange] Password Prompts 2010/2013 Coexist. Hi, I have a coexistence of 2010 & 2013 with some shared mailboxes and public folders being on the 2010 server but everything else on 2013. Some users are getting prompted for a username and password when connecting. I have NTLM set on the 2013 Outlook anywhere and basic on the backend 2010 outlook anywhere. Any ideas how I can stop this? Regards, Paul. ______________________________________________________________________ This email has been scanned by Selection Services Ltd using the Symantec Email Security.cloud service. ______________________________________________________________________
