It would appear that now the virus scanners are generating replies when they don't block a message:
From - Fri Sep 05 06:46:35 2003
Return-Path:
Message-Id: <200309041048.h84AmJ719287@outsmtp.ia.tablimited.com>
From: "InterScan Notification"
To: grantm@cpan.org
Subject: InterScan MSS for SMTP has delivered a message
This notification has been sent to inform you that a message has been
delivered by InterScan MSS for SMTP.
