son finds out the reason behind his mother’s strange disappearance every christmas when he receives a letter

The Letter

A few days after Sue’s death, when John came back home one evening he found a letter in his letterbox. He picked it up and threw it on his desk without even looking at the sender’s address. He just wanted to be with his father because it was his presence that could soothe his unrest soul. Who might have sent the letter and what was written in that letter?