Users seeing security warning when rebooting CAS server in cluster (Exchange 2010)

When rebooting a CAS server in a cluster, i've found that users will get a pop up security warning. To stop this, stop the Microsoft Exchange RPC Client Access service on all CAS servers that aren't being rebooted before doing the reboot. The downside to this is that all Outlook clients will show disconnected and won't download mail but users will not see any pop ups. Active sync devices will also not receive mail until these servers are restarted which you should do once the server you are rebooting comes back up.

After a reboot you should see the following services running for a standard exchange 2010 configuration (you may have POP3 enabled if you use it)