I would assume, that this could only happen with misconfiguration (if at all). Since you did not give detailts about your setup and the emails, nobody can tell.
asl send all the mail to a mail server that is in the internal LAN, this server works with postfix, when asl passes a mail to postfix and the user not exist in the domain, postfix rejects it with the following message:
450 : Recipient address rejected: User unknown in local recipient table; from= to= proto=ESMTP helo=
then asl keeps the e-mail in proxy content manager, and tries to send it frequently, always giving error in postfix, and the problem is that who send the message does not know that the user does not exist in our domain because asl does not warn it.
