Messaging Gateway - Virtual or Appliance - Licensing

I have a very specific question about licensing for the Symantec Mesaging Gateway (as below):

1) Can this product be used to resell (hosted) mail filtering? 

For Example: If i were to buy 500 Licenses would i be able to use those 500 licenses to provide message filtering (av/as) for 500 mailboxes from 10 different companies? The users who i am filtering for would work for different companies than me. 

I understand that symantec has a cloud based solution but thats not what i am talking about. 

Thanks, 

Robert