A business letter is a formal written letter which is sent from one business organization to the other. The characteristics a business letter depends upon the message it holds. The major feature of a business letter is that it transfers a business message from one organization to the other. Secondly, the general information enclosed in a business letter includes date of writing; return address, recipients address, salutation/greeting, message, closing and signature. Moreover, a business letter is free from grammatical mistakes, layman language and informal content.
A good business letter should state clearly in the first line what it's about. The letter should then stick to this topic, with separate points in seperate paragraph. It should be clear about what action you want to follow as a result of the letter (reply, delivery, whatever) and when. It should always be polite but not too chatty or informal. All this can also apply to an email.
Having a definite purpose