Scrutinize your corporate identity. Yes, how a company presents itself makes a difference. If you don't think this is important, your competitors will be thrilled. Corporate identity is the face you put on your company. Write customer-centered letters.
Most business letters have cold, impersonal words.
Yet, give the same executives a pen and they become stilted. Write as if you were the one reading it. But it may take two, four or six pages. A letter should be as long as necessary and always interesting to the reader.