When I make use of stress mode (select the check mark) with loadgen 2013 I do not get exceptions reported. However when I use non stress mode there are too many exceptions which are:

Microsoft.Mapi.MapiExceptionNetworkError: MapiExceptionNetworkError: Unable to make connection to the server. (hr=0x80004005, ec=2423)

 I am running loadgen on the exchange server itself and not a different client. 

Hi,

Stress mode just executes tasks as quickly as possible based on how quickly tasks are completing. It will ignore the day length and runs the tasks as quickly as possible.
The error when you select non stress mode, it may be caused by RPC issue. More details about it, for your reference:
https://social.technet.microsoft.com/Forums/zh-CN/b3c0eb67-cba6-4570-a563-ece482c3d06a/loadgen-2013-getting-mapiexceptionnetworkerror-unable-to-make-connection-to-server

Best Regards,
Allen Wang