I was working with a customer who had Exchange 2010 & were in the process of migrating to Exchange 2013. As part of their migration process they pointed their Exchange 2010 Outlook Anywhere namespace (let’s call it mail.contoso.com) to Exchange 2013 in DNS. At this point all of their Outlook Anywhere clients should have been connecting to Exchange 2013 & then been proxied to Exchange 2010. While this was somewhat working, they also immediately noticed users were randomly being prompted for credentials, resulting in a negative user experience.
Sometimes the prompts would be when connecting to Public Folders while other times mail or directory connections from Outlook to Exchange.
When I was approached with this issue/symptom it sounded familiar. After a search through my OneNote I realized I previously had a discussion with some people I know from Microsoft Support regarding this issue. Turns out this issue was recently addressed via http://support2.microsoft.com/kb/2990117 “Outlook Anywhere users prompted for credentials when they try to connect to Exchange Server 2013”.
This is actually an IIS issue with Server 2008 R2 (the operating system Exchange 2010 was installed on) that’s resolved by a hotfix. After installing the hotfix & rebooting the issue was resolved & their users no longer received the prompts.