The tone of the letter should correspond to the nature of the issue at hand. If a customer wrote to express his great satisfaction, a friendly, enthusiastic response is in order. If, instead, the client communication had a negative tone, the response should be businesslike but polite and should invite further communication to ensure concerns are fully addressed. Proper grammar and spelling are de rigueur, whether you respond by mail or email. A letter or message rife with misspellings makes you look incompetent and may encourage a customer or client to take his business elsewhere.