Using Microsoft Exchange 2010 SP1 or SP2 with roll up packs installed. The RPC Client for Microsoft Exchange is taking up large amounts of CPU causing your exchange server CPU to hang at 100%.
This could be due to a couple of reasons, one is Pubic Folders and the second OAB, so this would be a starting point.
If you don’t have a Public Folder store, create one and monitor the CPU whilst you mount it. If CPU decreases but starts to increase again, dismount the Public Store and leave dismounted. CPU should start to stabalise. You may need to restart the Microsoft Exchange RPC client service.
Also, if you are using Citrix/Xenapp, if you find that lsass.exe takes up large amounts of CPU, this may resolve the issue after the Citrix servers are rebooted.