Recently I worked on an Exchange 2007 to Exchange 2013 on premise migration. We had cut over the client access services from Exchange 2007 to 2013. Client Access was published externally via IIS ARR. I used this post by my good friend (and Lync Guru) Yoav Barzilay to configure IIS ARR. Although the post is for Lync, it is a great guide that will also apply to Exchange.
Everything was working fine, except external Outlook Anywhere clients. Outlook would prompt for login and then would remain “Trying to connect” after the login credentials were entered.
As the issue only affected external clients, and in order to narrow the troubleshooting, I decided to temporarily bypass the IIS ARR server and NAT our client access services directly to Exchange. Outlook Anywhere was working in this configuration. So I knew I had an issue with IIS ARR.
In the proxy settings of each of the server farms, I changed the “Response Buffer Threshold (KB)” from the default of 256 to 0. After an IISRESET, Outlook Anywhere worked like a charm J