How to write important letter

Following are the things that must be kept in mind in order to write an important letter;

The address to where the letter is to be sent must be accurate. Any mistake, and the letter would go to waste.The problem should be mentioned in the subject before starting the letter.The letter should not consists of metaphors or any irrelevant information as it would lose the purpose.The issue at hand should be presented at the start and should be discussed in detail for the reader to get an idea of the situation in depth.