You know the person personally to whom you're writing
Exclamation marks allowed
Slang allowed
Address and date and signature
EXAMPLE
Maya White
33, Downing Street
London SW1A UK
6th January 2018
Dear Maya,
Thank you very much for your last letter. It was great to hear from you after so many months. You seem to be having a nice time in Paris.
Thanks also for the photographs. I absolutely love the snap of yours standing in front of the Eiffel Tower. Paris looks stunning! Someday I would definitely like to go there.
There's not much happening here. I'm busy with final exams at the moment.
By the way...are you coming home anytime soon? If you are, let me know the dates and we can arrange to meet up.