Seeing as Dovecot recognises when a virtual recipient is invalid, couldn't it somehow put the to-be-rejected mail back to Postfix (or whichever MTA)? Or should it just drop it? I don't know what the SMTP (whichevery protocol is to be followed here) says about this, so I pose the question again:
Once you have accepted the mail *YOU* are responsible for dealing with it and generating bounced back to the original sender as per all the RFCs. If you accept obvious forged mail then this is going to get you sending bounced back to poor innocent forged recipients and listed as a spam sender when you hit spam traps...
Just don't accept mail into postfix that you don't want to have to deal with later... Kill it right at the front door and reject it and then it becomes the upstream server's problem to deal with...
Good luck
Ed W