This article below explains the cause although it is referring to the SMTP log entry, not specifically what the user sees on the mail client: http://support.ipswitch.com/kb/IM-19980116-DD02.htm
Sue K Please visit http://www.ipswitch.com/support/mailing-lists.html to be removed from this list. An Archive of this list is available at: http://www.mail-archive.com/imail_forum%40list.ipswitch.com/ Please visit the Knowledge Base for answers to frequently asked questions: http://www.ipswitch.com/support/IMail/
