Your letter needs to describe Holden and his personality, and it needs to be written from a character within The Catcher in the Rye. Your first task is to pick which character will write the email letter. I recommend picking a character that knows Holden well. That eliminates a lot of characters right away. You could write the letter as Sally, since Holden and she are kind of an item. At least Holden believes they are close enough to perhaps run away together. You could also use Mr. Antolini. He's not in the novel very much, but it is clear from the beginning of the book that he has Holden's personality pegged. Probably the best character to use is his sister, Phoebe.
The problem with using Phoebe is that she is ten years old. She's not likely going to write a very formal email very well. On other the hand, Holden thinks she is one of the smartest people that he knows. Mr. Antolini is going to be able to write a very formal email, because he is an English teacher. The problem is that he only sees Holden in class, so his point of view is much more limited than Phoebe's or Sally's point of view.
For the formal email part, be sure to start with "Dear" and end with "Sincerely." Proper punctuation is going to be critical. I recommend staying away from all contractions; write "do not" instead of "don't." I'm going to suggest picking three parts of Holden to discuss. Each of those should be its own paragraph. Do not string everything together in one giant blob of text.
Your prompt says that the letter needs to stay in the third person, so do not write "I" and "we" in the letter. You can't write something like "I have known Holden for 10 years." Skip it completely, and just write to be informative about Holden. "Holden is an interesting big brother. He cares deeply for truth and honesty, which is why he can not stand people with a phony character."