Mail that is registered by the post office when sent in order to assure safe delivery WordNet 3.0 & Open English WordNet

Registered mail is a mail service offered by postal services in many countries which allows the sender proof of mailing via a mailing receipt and, upon request, electronic verification that an article was delivered or that a delivery attempt was made. Wikipedia

Letters, packets or other postal documents considered valuable and in need of a chain of custody Wikipedia Disambiguation

Postal service Wikidata