When replying to a letter or inquiry from clients, associates, customers or colleagues, it is important to maintain an appropriate amount of formality. Your written communication may very well be the first and possibly only correspondence between you and the original sender, so making a good impression is crucial.
Open your reply letter by making reference to the original letter you received. Use an opening line such as "With reference to your letter" or "Thank you for your letter/e-mail." At this time, paraphrase some of the key points mentioned in the letter you received. This lets your contact know that you have grasped his message and understand his position. This is an important element if you are dealing with a boss or customer, and even more important if it is a boss or customer issuing a complaint. Remember not to use his first name unless you are close friends or colleagues and he used your first name in the original letter. For someone you do not speak conversationally with, formal is best. Use the last name preceded by Ms., Mrs. or Mr. if you know the person's name; and if you do not know the name use "Dear Sir or Madam."
Follow through after your introduction by addressing the question, inquiry or complaint. Answer any questions or direct the person to someone who has the answers and do your best to solve any problems brought to your attention. In the case of a complaint, be sure to apologise for the issue at hand. Do not get defensive or rude about the matter, but state your case in a polite and non-judgemental way. You can use a phrase, such as "I apologise for the inconvenience. I would be more than happy to speak with you to find a resolution to your complaint. Your business is valuable, and I hope to keep you as a customer."
Close your letter by using a phrase that anticipates future events, apologises a final time or offers help. Some common phrases that are acceptable include, "If you require more information, don't hesitate to contact me," "Once again, I apologise for any inconvenience" or "I look forward to your reply." Again, maintain formality as you finish the letter. For a recipient you do not know, or if you are addressing an entire department use "Yours faithfully" followed by your name. If you do know the name, use "Sincerely." For a friend or close colleague, "Best Wishes" or "Best Regards" are both appropriate.