We have a customer that uses thin clients and multiple RDS servers for the majority of their users. They use a software application that contains a feature which will open a new email message window from within Outlook to simplify the process of sending an email when needed in their workflow. When the customer was running Exchange 2010, this feature worked correctly, but after upgrading to Exchange 2013, the new email message window does not open in Outlook when prompted from the other application. Instead, the user is prompted by Outlook to enter their credentials. Whether the user cancels the prompt or enters their credentials correctly, authentication fails and they are immediately prompted for credentials again, over and over.
We have ruled out anything specific with the third-party application. We know it works fine with other customers on Exchange 2013 (and other customers using RDS servers) and for the users at our customer site that have full PCs that are not using the RDS servers. Only the RDS users at this customer site face this issue. We also known the same issue occurs if the RDS users click any "MAILTO" hyperlinks or shortcuts, thus eliminating the third-party application from the equation.
The customer is running 4 RDS servers (WS 2008 R2) and a separate Exchange 2013 mail server within their network. The users have Personal Desktop VDIs running Windows 7 with Outlook 2010 in "online" mode (it is worth mentioning that the same credential prompt behavior occurs in "cached" mode as well, but the email message windowwill open after credentials are successfully entered). When running a network trace, we see that there are a number of HTTP 401 errors with authentication going to the Exchange server (this occurs if authentication is set to "Basic", "NTLM" or "negotiate"). Again, this was not an issue while on Exchange 2010 and non-RDS users do not have this issue while now on Exchange 2013.
Perhaps the issue is with the user profiles in the RDS environment (registry values for Exchange/Outlook need to be updated somehow)? Perhaps the RDS sessions cannot communicate properly with Exchange since 2013 switched to using RPC over HTTPS? Perhaps the RDS user logins are not set properly for the correct mail profile to be identified/assigned/authorized? We are trying to advise the customer where the issue lies, and could use some advice.