You shoud probably start with the Postmaster for the post office where the letter was sent.
This answer is being provided for general educational purposes only and does not constitute legal advice. No attorney-client relationship has been established. This answer should not be used as a substitute for competent legal advice from a licensed professional attorney in your state.
Christopher S. Tretola, Esq.
The Spadaccini Law Firm, LLC
98 Franklin Corner Road
Lawrenceville, New Jersey 08648
tel: (609) 912-0100 x 21
fax: (609) 912-0400