I have to admit that when I drew up lists of agencies to partner with I did not consider the U.S. Postal Service as one that even came to mind. However, as an article in the current edition of Emergency Management Magazine points out Letter Carriers Add Bioterror Response to the Postal Service
The mail connection became more evident with the anthrax letters that followed the 9/11 attacks. There were plenty of precautions being taken around the United States and that is when I got introduced to my Postal Service counterpart for the Puget Sound Region. As noted in the article about using letter carriers to deliver preventive medication (antibiotics) via the mail the concept was tested in the Seattle area. It would seem to work if:
We also figured out that it will be very difficult to provide security for every letter carrier. There just are not enough law enforcement resources to do so. The issue is that once it is known that the drugs are coming via the mail, desperate people might go after the carriers while on their routes to either obtain the drugs for themselves and their family or to black market them to make a buck.
The U.S. Department of Health and Human Services has a good Emergency Preparedness website that provides a wealth of plans and other guidance.