noun
forged letter
A letter that has been falsified, typically to deceive the recipient about its origin or content.
彼は偽手紙を使って会社の評判を落とそうとした。
He tried to damage the company's reputation using a forged letter.
Compound of 偽 (にせ, 'fake') and 手紙 (てがみ, 'letter').