The Best Punctuation Book, Period Quotes

“A common comma error occurs when an e-mail or letter greeting is structured: Hey Jane, Hi Pete, Hello everyone, Howdy stranger, Jane, Pete, everyone, and stranger here are direct addresses that should be set off with commas. Right: Hey, Jane. Right: Hi, Pete. Right: Hello, everyone. Right: Howdy, stranger. Note that these greetings follow a different grammatical structure from the classic Dear John, or Dear Sirs, in which the word dear is an adjective and therefore part of the direct address (part of the noun phrase). Unlike hey and hello, dear is not a complete thought. So it makes sense to follow Dear John with a comma, thereby integrating it into the first sentence of the e-mail or letter. But Hey, Jane and Hi, Pete are complete sentences that can be followed by periods or other terminal punctuation.”
― June Casagrande, The Best Punctuation Book, Period: A Comprehensive Guide for Every Writer, Editor, Student, and Businessperson