Your analogy is a slight bit confusing, but what it sounds like your seeing are the mailboxes in Exchange. When you delete a mail enabled user from AD, Exchange retains the mailbox for 30 days by default. As for message tracking, your seeing the path the message took i.e. user1 is the primary recipient and user2 will receive forwards from user1. Note that once you have deleted a user from AD, re-adding that user does not automatically link the original mailbox to that user account even if you rename the user exactly as it was originally. You can however manually link a mailbox to an account through Exchange System Manager.