While a simple "Hi," "Hello," or even "Hey" is appropriate in casual correspondence, a more formal salutation is appropriate when you are emailing about a business_related matter, such as a cover letter, letter of recommendation, or inquiry letter.
Application letters and cover letters. Many people believe that application letters and cover letters are essentially the same. For purposes of this handout, though, these kinds of letters are different. The letter of application is a sales letter in which you market your skills, abilities, and knowledge. A cover letter, on the other hand, is primarily a document of transmittal. It identifies an item being sent, the person to whom it is being sent, and the reason for its being sent, and provides a permanent record of the transmittal for both the writer and the reader.