Windows 2003 SP2 and Exchange Serve 6.5 SP2 mails redirects

In our organization we run windows 2003 SBS SP2 and we also have an Exchange server 6.5 build 7638.2 SP2. We are however experiencing some errors – some mails are not sent to the intended target but instead get redirected to the admin. Can you help me understand why mail intended for a particular user is sometimes sent to a monitoring account in a different domain? I checked the message tracking center and there is no trace of it. Also, the incoming mail fails to appear in the log. We have also deactivated any type of filtering. I have run out of options and would like your help in figuring out this matter.
