you can certainly do this, and if i'm not mistaken the setting for this would be in the global settings -> MTA of the admin panel. specifically the "relay MTA for external delivery" setting.
however you can do the same thing with zimbra by installing just MTA (postfix) and AV/AS services on said machines and maintain the integration with the rest of the system. fairly certain you can put AV/AS on its own machine and MTA on its own machine if you needed too.
*edit* forgot the link
Improving Anti-spam system - Zimbra :: Wiki