Does business etiquette require you to send a “thank you” letter in response to a letter from a client complimenting you on the service that you have provided them?
The key word in your question is “require.” No, business etiquette wouldn’t require you to send a thank you note. However, I would certainly recommend it. It’s your opportunity to reach out and touch someone who is satisfied with your work. Writing back will help you build a more solid relationship with this customer. That’s why you make the effort; not because business etiquette requires it. Something as simple as, “Thank you for your kind words about the service we provide. We take great pride in building the best relationships with all our customers. It wonderful to know our efforts are paying off. I hope we can be of service again in the future.”
Source: Post, Peter. "Etiquette at Work." Boston Globe 14 Oct. 2007.