noun
contents-certified mail
A postal service that certifies the content of a mailed document, often used for legal notices.
契約解除の通知は内容証明郵便で送った。
I sent the notice of contract cancellation by contents-certified mail.
Compound of 内容 (ないよう, 'contents'), 証明 (しょうめい, 'certification'), and 郵便 (ゆうびん, 'mail').