I believe you are trying to install SEP client on a exchange server. See the below link. SEP has automatic exclusions for exchange 2010 if you are running RU6 MP1 or later. You don't have to do anything special.
Preventing Symantec Endpoint Protection 11.0 from scanning the Microsoft Exchange 2010 directory structure
http://www.symantec.com/business/support/index?page=content&id=TECH97707
If you are trying to install SEP as your e-mail protection, then as mentioned above, the right product is Symantec Mail Security for Exchange Server.
Hope this helps.