Microsoft provides great testing tools for Exchange on so I decided to run EWS connectivity test from the toolset. What I didn’t know at this point was what was causing the server to respond with this error. The error is somewhat misleading as when you look at EWS logs on Exchange server, you will see that the server responded to the request with HTTP error 400 which translates to “bad request”. Mac Mail was throwing “Account Error” which read “Unable to connect … The server returned error: Connections to host on default ports failed.” It is worth pointing out that while Outlook 2013/2016 on PC use MAPI/HTTP or RPC/HTTP to connect to Exchange 2013/2016 servers, both clients on Mac (Mac‘s own Mail client and Microsoft Outlook for Mac) uses Exchange Web Services (EWS). Latest Microsoft Outlook for Mac did not exhibit any issues either! Microsoft Outlook 2013/2016 clients on PC did not exhibit any issues. The connections to the Exchange servers were also encrypted.
KEMP LoadMaster was configured to terminate SSL connection on the load balancer. The Exchange environment was mixed Exchange 2013 and Exchange 2016 but load balancer was configured to use only Exchange 2016 servers. The load balancer was configured to user Layer 7 load balancing. On his Mac, built-in Mail client would not connect to Exchange through KEMP LoadMaster. My good friend and renowned Exchange MVP Jeff Guillet ( fame) brought an interesting issue to my attention.
