American EnglishAmETCM a way of sending post in which someone records that you have sent it, and the person it is sent to must sign their name to prove they have received it 挂号邮件
Examples from the Corpus
certified mail• It also sent the memo by certified mail, with return receipt requested.• The Giants suggest sending the tickets by certified mail.
From Longman Business Dictionary
certified mailˌcertified ˈmailAmerican EnglishAmE
a postal service which, for an additional payment, provides proof that a letter or package has been posted and deliveredSYN BrE RECORDED DELIVERY