Postmaster: hi dear (You can even write the name of the daughter)
daughter: Hello father!
P: What happened to you? Why didn't you send me any letter from your side, but above all tell me about your health... how are you feeling now?
D: Not so good, father! I was not in the condition to write any letter to you, that's why I didn't write any letter to you. I'm really sorry, father. I know how worried you must have got.
P: What are you saying? You are my daughter and don't ever say sorry to me, I can understand your situation, it's just that I was little scared and concerned about you.
Ok bye, dear and make sure to take good care of your health.
P: Ok father. Even you make sure to take care of your health and don't worry about me. I'll be fine. Bye.