noun
certified mail; content-certified mail
A postal service that certifies the content of a mailed document, often used for legal notices. The full term is 内容証明郵便.
契約解除の通知を内容証明で送った。
I sent the notice of contract cancellation by certified mail.
配達証明 certifies delivery, while 内容証明 certifies the content of the mailed document.
Compound of 内容 (ないよう, 'content') and 証明 (しょうめい, 'certification').