Remaining formal in business writing until a relationship has been developed with the receiver of your letter is always the best policy. From my experience, the receiver of your letter will eventually reveal to you when your business relationship has gone past the formal stage. Never become familiar with people until you are given the clue by them to do so. Make sure your letter is appropriately marked—sensitive, confidential or personal to have the letter read by the intended person. This would be the same etiquette if you received a letter. However the letter is marked; make sure that it is placed in the hands of the right recipient. Keep in mind that it is a good practice to maintain a professional or formal style with good business etiquette even with people that are familiar because some one else may be privy to the letter. And lastly, I think its good etiquette to personally sign the letter and not sign with an electronic signature.
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