What was wrong was I didn't follow the deployment guide like I'd previously done when I didn't use the Exchange template, the template had made me lazy as it does all the hard work for you. But there are some basic settings that need to be changed on the load balancer so that the load balancer will function correctly.
In my case I hadn't change "Drop Connections on Real Server Failure", this is a very quick change on the Web User Interface (WUI), from the Kemp deployment guide:
By default existing connections are not closed if a Real Server fails. This can lead to issues with Outlook clients if an Exchange CAS server fails. A solution to this is to enable the Drop Connections on RS Failure option which can be found on the System Configuration > Miscellaneous > L7 Configuration screen in the WUI.
When this option is enabled, LoadMaster tracks all the incoming connections and which Real Servers they are connected to. When a Real Server fails, all connections to the Real Server are immediately dropped, forcing the connections to reconnect to a different Real Server.
Enabling this option has the added benefit of allowing relatively higher Idle Connection Timeout values to be set as the danger of the client retaining a connection to a failed server is removed.
Figure 1: L7 Configuration
So always make sure that you read the deployment guide and apply the settings required even if the templates do all the hard work for you…
Kemp Exchange 2010 Deployment Guide - https://support.kemptechnologies.com/hc/en-us/articles/201737917-Exchange-2010
