The general format for a business letter is an opening salutation followed by the body of the letter and finalized with a closing. However, there are dozens of potential closings that may be used to end a business letter. Before choosing the closing that is right for your particular letter you should consider the tone of the letter as well as your personal history and past interactions with the letter's recipient.
Business letter closings such as “Sincerely,” “Very Truly Yours” and “Respectfully Yours” are considered to be formal, impersonal closings and are commonly used when sending business letters to recipients with whom you do not have established personal relationships. Using one of these formal closings in a letter to a business associate with whom you have a friendly relationship is acceptable; however, it may seem somewhat rigid and impersonal given the history of your relationship.
Semi-formal closings such as "Best regards," or "Cordially" are often used when sending business correspondence to a lateral associate with whom you have an established rapport. A semi-formal closing is not usually appropriate when sending a letter to your boss or a new business contact as it may be viewed as too personal or uprofessional.
If you maintain a friendly, casual or lighthearted relationship with the recipient of your letter it is appropriate to use closings such as “Best Wishes” or “Best of Luck” when concluding your letter. Again, these are relatively casual, nontraditional closings that should be reserved for friendly business associates rather than formal business contacts.