The letter format should use an appealing font. Maintain uniformity of font and size all through the body of the letter. At the same time, you should create headlines and titles to seem different by simply increasing the font size slightly. Capital or bold letters should be done with care so that reading continues to be smooth. The art of sales writing is time consuming. You could refer to a few sample letters to see how they have made good use of fonts to give a perfect finish. The sales letter format should emphasize key points while maintaining simplicity. Do not repel the customers or clients. Have normal spacing between lines and the contents just right so that the reader does not feel over_burdened reading your letter. Get the reader to feel as if he is invaluable to you and your company. The pronoun "you" in place of "we" instills a sense of warmth in the reader, drawing them closer to your business.
A resignation letter is mostly sent as an email and must be addressed to the reporting manager and the HR. It's not just an information for your boss that you intent to leave but it is also an advance intimation to superiors, and human resources so that they can find a replacement for your position.