Moved this to our ePolicy Orchestrator area.
Move the exchange system to a separate folder (for easiness). there are other ways to do this with tags and all.
then apply the virusscan client task to this folder (if the task is inherited, then no need for this).
under policy tab, select McAfee virusscan product. in the list of policied, break the inheritance of 'On-Access Default Processes Policies' and edit the policy to add exclusions.
I would't exclude drives; I rather choose the files to exclude such as the exchange database file and log file.
excluding drive or folder is risky