Can The Lanlord Go Into My Closed Locked Mail Box And Put A Certified Clerks Office Letter That Was Sopposed To Be Mailed Or Hand Delivered ?

Well I guess he can, if he did? At least physcially BUT

Why or how does he have a key to your mail box ?
Does he put the mail into the boxes.

Please explain more about the issues, what type of letter ? Was it from him or was it from the court ?