To expand on this problem I think it is possibly this:

http://www.nabble.com/Spam-with-Delivered-To-header-t3689885.html

I'm just not quite sure how to go about fixing it. Could someone give me a few
more details. How do you use the script that they are talking about?

This seems like what I have: postmaster sends me a user unknown email. Inside that email is another email that is addressed to a known user, from an
unknown user that was sent from the post master to the unknown user saying This is a delivery status notification from icarus, running the Courier mail server, version 0.47.

The original message was received on Fri, 19 Oct 2007 00:16:55 -0600 from danielpwns ([::ffff:98.18.15.171])

---------------------------------------------------------------------------

UNDELIVERABLE MAIL

Your message to the following recipients cannot be delivered:

<know...@mydomain.com: <<< Mail loop - already have my Delivered-To: header. AND attached to that email is a spam email... from unknownuser@mydomain to a
known user at my domain.