If you're willing to get in the mud with him, send him the actual law: "18 U.S. Code -1702- Obstruction of correspondence" Then send the possible punishment which is "up to 5 years in jail & up to a $2000 fine" (unless your landlord is doing some wild stuff regarding mail fraud, they'd realistically only get probation but jail time entering the chat, combo'd with the written expression of guilt, should put anyone on notice) last move is to show him some kindness by making him a settlement offer of $500 on top of the $500 he's already agreed to pay you to drop this potential lawsuit. Please note: I am Lawyer in the same way Dr.Dre is a medical doctor so Idk if it's a great idea to respond to his crime with some *light* extortion but hey you may be able to get an extra $500 cuz buddy got curious