Longman Dictionary of Contemporary English
certified mail
noun
EXAMPLES FROM CORPUS
▪ It also sent the memo by certified mail, with return receipt requested.
▪ The Giants suggest sending the tickets by certified mail.
Wiktionary
certified mail
n. (context US English) first class mail that provides the sender proof of mailing and a delivery record.
Usage examples of "certified mail".
We sent it first-class, certified mail so you have to sign for it.
Copies of all the pleadings had also been sent via certified mail to the Phoenix.