It is reported that MAPI over HTTP which is a newer connection method of laterExchange servers and potential better and more reliable for devices connecting has some unreliabilities in some instances with earlier version of Outlook. The CTRL right click on the Outlook connection icon (bottom right) showed the connection was made with HTTP The user reported that outlook was slow to open email, and unresponsive with searching in outlook. Outlook 2013 connecting to Exchange 2016 with MAPI over HTTP enabled.

In our case the versions in question were found to be:

Outlook Slow and Unresponsive wiht MAPI over HTTP to Exchange 2016 Server