Problem ex-tenant

Other

We recently had a tenant move out of our house when the 6 month agreement ended. The house was in a disgusting state and it cost us loads to get it habitable again. Unfortunately we're now having threatening letters from debt collectors from before he moved into our house. We contacted the ex tenant and he's now threatening to take us to court for opening his letters? The letters stated they should not be ignored so we feel we had no choice, is it really illegal to open letters sent to our address which are clearly urgent. He left without giving us a forwarding address but we still had his mobile number.
R and T