If you use Cluster Services with Microsoft Exchange Server version 5.5, you do not need 
the Roll-up hotfixes, but you must install the Cluster hotfixes:

   ftp://ftp.microsoft.com/bussys/winnt/winnt-public/fixes/usa/NT40/hotfixes-postSP3/roll-up/cluster/

WARNING: Do not install the Roll-up hotfix if you have previously installed the Cluster
hotfix.