Well-written business correspondence will contain all the elements of politeness, professionalism, and clarity. There is nothing wrong with applying any of the traditional phrases commonly used in formal correspondence. These get overlooked these days because e-mail habits have become too casual. Business correspondence falls into the category of legal document and it should be treated as such. Maintaining a good format for your business correspondence is very easy. For as valuable as standard phrases and accepted terms might be, there is nothing to stop you from personalizing your messages while maintaining professionalism. You are always free to design your correspondence any way you wish.
The composition of a good business letter should always convey appreciation, respect, concise clarity of your message, and appropriate references to related meetings, documents, and correspondence - whatever the case may be. If you research business letters online, you'll find a large quantity of examples that address how correspondence in various business organizations is handled. These are a great source of ideas for letter formats and phraseology.
Business correspondence composed by you is a direct reflection on you personally and professionally. You own it, so take advantage of the fact that you are in charge. Let your correspondence reflect you, whether ultra-formal or friendly/casual. A combination of the two is most common in American business. Internationally, correspondence should be more formal.
Your relationship to your client should dictate the tone of your correspondence, but keep i mind that what you write is a legal document. Don't be afraid to give your personal touches and preferences. If a suggested term or sentence doesn't feel right to you, just scrap it. I often close letters with the word "Respectfully,". I also still use "Dear..." as a salutation. It's very traditional, but it works. All of the variables are your choice.
