Joe Topjian has written a two-part tutorial about virtual domain support in Postfix. In his tutorial he uses separate domains and non-unix accounts. He writes:
"This option gives us the most flexibility. It's a little more complicated to set up and understand but well worth it when you're hosting several domains.
Our end goal is to have an email server that supports mail delivery to multiple domains. Each email address will be authentic to only that domain. For example, [email protected] and [email protected] are two different accounts that each receive different mail."