Taehyung writes letters to himself and mails them to the most random, fantastical places he can think of, with the knowledge that no one is ever going to receive them.
Jeongguk just so happens to be the mail boy.
"You're not running away this time Jungkook."
Kim Taehyung gets a letter by "J.J.K" each day with his thoughts of the day and deepest secrets. He knows everything about him through the letters, but not his full name or address.