Landlord theatened to destory me and my other tenant's mail in the future if it accidentially arrives at neighbor's place agian. is this illegal?

I share a rental house with 3 other people(divided apartments). It is technically an illegal rental we do not have leases and thus potentially less legal rights. Accidentally, the other tenant's mail has arrived at our neighbor' house. Landlord has expressed frustration and instructed us to gain PO boxes. I did not yet have one. Had to have an emergency/legal letter sent to me. Under the stress of a legal situation days ago accidentally put in my neighbors address not mine. This morning was informed it arrived at her place. I picked up mail apologized to the neighbor, explained my mistake of mine and assured would never happen again. In a Facebook group of me. tenants and our landlord. Our landlord proceeded to threaten that if this ever happens again she instructed the neighbor/the landlord herself potentially will "destroy" our mail. I was informed by a fellow tenant this is illegal which prompte me to reserach. my question is threatening to destroy a tenant's mail/property illegal?