Missing Active Directory Account for Mailbox

We have an Exchange 2007 environment running on an Server 2008. I recently found a mailbox in the Exchange Management Console pointing to an Active Directory account, but the user account isn't showing up in Active Directory User and Computers snap-in. The mailbox is used for a monitoring computer and it is able to log in (I found the password for it). How can a mailbox exist in Exchange 2007 without an Active Directory Account? And now can the user log in to access the mailbox without an Active Directory Account?

This one has me scratching my head.

Thanks

From serverfault Edison

• There has to be an account. Exchange aside, if you're logging-on as the user there's an AD account.

  Try a query with the dsquery tool and see what you find:

   dsquery user -samID account-name
  

  That should return the DN of the user object in question.

  Edison : Thanks for this. I was able to find the user in a buried OU within AD. Thanks again.
  From Evan Anderson