If you are no longer using mail for your Virtual Server, you’d want to make sure you go into Edit Virtual Server, and disable the “Mail for Domain” feature – otherwise, it’ll still try to deliver those locally.
I actually think that there’s another setting required for this to work properly. When I changed the mail handler to Google Apps, simply disabling the ‘Mail for Domain’ option didn’t entirely solve the problem - mail was still delivered locally.
You have to go to Webmin > Servers > Postfix Mail Server > Virtual Domains and remove the domain(s) which you have migrated to Google Apps. For some reason, disabling Mail for Domain does not remove this entry.