Do not write: I am in receipt of your letter of 10.10.2019 . . .
Write instead: Nothing. Unnecessary.
Here’s why: If you weren’t in receipt of the letter you wouldn’t be answering it.
Actually, this is a long ago response, sometimes written as, “I am in receipt of yours . . . ” It’s as though the writer doesn’t know how to begin. Gratefully we don’t see it often any longer.