Postfix local delivery for external accounts

Hey,

Sometimes existing clients decide the host their email in there office and ask us to point their MX record to the new ip.

So what we do is disabled their mail in virtualmin and create a new MX record in their DNS.

The problem is if they leave some old mail accounts in virtualmin, postfix tries to delivery mail locally. Sendmail didn't behave this way. If the domain was not listed in local domains it would ignore local accounts.

It appears that postfix will try and delivery the mail to a local account/alias even if the domain is external.

Is there a way to change this behaviour similar to the way Sendmail deals with non local accounts?

Cheers,
Ryan