Create your virtual domains by adding them to the virtual file in the /etc/postfix config directory.
Once you create or edit this file, run a command like this so it updates it to be readable by postfix:
postmap /etc/postfix/virtual <virtual
The contents of the file for virtual domains for mail will look something like this:
Where the first line indicates the domain name and states that its a Virtual domain..
The second line is the email alias that points to the local user account, etc, kind of formatted like the aliases file.