Symantec Corporate and Mail security for Exchange

Hi,

I currently have 20 licenses for Symantec Corp Edition. We are installing an Exchange server.

Should I upgrade to the Enterprise Edition of Symantec and buy 20 upgrade licenses or can I buy 1 license for Mail Security for Exchange (for the server).

And rely on the Symantec Corp Edition client to catch the viruses using “Microsoft Exchange Auto Protect” accessible from Configure menu?

Thanks