Email being sent to unknown users

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From: Lee (leemack33_at_hotmail.com)
Date: 03/31/04 

Date: Wed, 31 Mar 2004 10:42:52 -0800

We just migrated to Exchange 2003 and we are seeing some
of our users get bounced emails from users that they did
not send email too. Any ideas? Exchange 2003, and using
Outlook 2000, XP, and 2003.