Uri Chinhaejil Su Isseulkka

Ji-Tae finds himself in crisis because of Tae-Heon, who succeeded in a massive gang bust operation. After all his collection agents and even his sex partners were arrested by prosecutors, the lone Ji-Tae, consumed by vengeance, sends Tae-Heon countless letters containing sexual harassment to torment him. Finally, with his work disrupted, Tae-Heon, filled with rage, checks the sender of the letters... The sender is his high school classmate, Lee Ji-Tae?!

(Source: Lezhin, translated)

Note: The third story in Lezhin's Jungnyeon Danpyeonseon (중년 단편선) Collection.