Seen on Cryptome.org:
DoJ and DHS — which should know better but may pretend innocence — use govdelivery.com, probably others. We have complained to the service before without response.
To see the tracking code look at the HTML code underlying the links. Clicking a link will produce a 404.
This was discovered by use of an old email program. New programs tested go right to the linked file and do not disclose the tracking taking place.
Read more on Cryptome.