Using standard letter closings which have been used since ages is not incorrect, but its appropriate use is a must. For example, 'sincerely' seems to be too formal a word to be used in a personal letter and also in certain formal relationships where you know the client or the business partner for a fairly long time.
There are not many formal letter closings as compared to personal letters, so, you could try to use salutations that sound friendly yet formal. Also, consider the occasion of writing the letter. While personal closings help in expressing warmth, professional ones should suggest appreciation, respect, and sincerity. The examples that you are going to find in this article will provide you with a variety of closings to choose from.
The list of personal closing lines can practically never stop and can go on and on. What's even more interesting when it comes to informal letters, is that, you can try to come up with your own closing lines and add an extra personal touch, which would be exclusively yours.
Complimentary closing remarks are an integral part of business letter-writing etiquette. These examples can be used when writing letters of a professional intent.