You may receive multiple prompts for authentication from Outlook clients connected to an EBS Messaging Server at roughly 5 minute intervals.  Both local and Outlook Anywhere clients can encounter this issue, but it is usually more pronounced on OA clients. The behavior may be inconsistent for different users and is remedied temporarily by rebooting. To resolve this issue, log on to the Messaging Server directly and run the following commands from an elevated command prompt (commands may wrap in this post, you may need to combine the lines from copy and paste):

%windir%\System32\inetsrv\appcmd.exe unlock config -section:system.webServer/security/authentication/windowsAuthentication

%windir%\System32\inetsrv\appcmd.exe set config "Default Web Site/ews" -section:windowsAuthentication -useKernelMode:False /commit:apphost

%windir%\System32\inetsrv\appcmd.exe set config "Default Web Site/AutoDiscover" -section:windowsAuthentication -useKernelMode:False /commit:apphost

%windir%\System32\inetsrv\appcmd.exe set config "Default Web Site/OAB" -section:windowsAuthentication -useKernelMode:False /commit:apphost

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