Exchange IMS mailbox.

Here is a problem.
On our exchange server in the server admin mailbox beside the usual undeliverable email notifications I found some emails that were send from a single user to another user. That email is being copied into the admin mailbox even though admin is not on the delivery list. Also if the sending party would mark the email for delivery confirmation he would get additional receipt from the admin mailbox. Had anyone experience anything like that and can something be done to cure it. The setup is Exchange Server 5.5 with Outlook clients.