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.
Return address and date: (Upper right hand corner) Once upon a time, this was necessary in case the letter became separated from the envelope, but I doubt that happens much any more. Even so, this is where your reader is going to look to find your address and the date the letter was written, so it should be there. Mailing address of recipient: (Left_hand side, one line below the return address) This identifies the recipient of the letter. In office environments, letters are often removed from their envelopes and circulated; this helps the letter get back to its rightful recipient.