When a vserver alias is created postfix stops rejecting non-existing users

When a virtual server alias is created for another existing virtual server a catch-all entry is created in postfix's virtual dbase that points to the existing domain:
@alias.domain @existing.domain
This is a security issue because postfix (if I am wrong correct me here) does not check the virtual table recursively to check whether to reject a user or not.
So all emails to non-existing users for the alias-domain are accepted and this creates much spam-scatter.
So the big solution would be to re-create all users of the existing domain for every alias that is added and to delete them when the alias is deleted.
If I am wrong about postfix and recursive check of the virtual table please do tell me so.
Thanks