Backscatter from virtual mailbox aliases

[huda]

Our virtual mailbox is: domain.com.au
It has an alias: domain.com
Emails sent to bogus@domain.com.au (bogus is not a user in the domain) are rejected.
However, emails sent to bogus@domain.com are bouncing back, thus contributing to backscatter.
It seems the problem is that the map for virtual alias domains is returning a catch-all alias, rather than there being explicit aliases for each user in the virtual domain (so Postfix can reject the mail in the SMTP session).
Any ideas how this can be fixed?