I was recently alerted to a large number of outbound messages in the mail queue. On checking I found the sender listed was not in the GAL or even from our domain. Relaying is set to the local server and authenticated users. How can I find the true source of the e-mails and prevent future problems. Using SBS 2003 with Exchange 2003 SP1.

Are the messages in the queue for inbound recipients from senders outside the domain? You probably want to make sure you aren't experiencing an NDR Attack (http://support.microsoft.com/kb/886208).