Dear John. Dear, dear John. If I were to write a Dear John e-mail, I would start it with:
If I were to write a business e-mail to John, I would start it with:
You probably already know that.
What you may not know is that if I were to write a friendly e-mail to John, I would start it with:
Why the “extra” comma? I know it looks strange, but Hi, John, is actually correct, because dear and hi are not the same kind of word so the punctuation is different.
Dear is an adjective that modifies John. John is dear to me, so I call him dear.
Hi is an interjection, like wow. When you say hi, you are directly addressing John, so the direct address rule comes into play.
In terms of comma rules, Hi, John is no different than, Whoa, John, slow down. “Hi, John,” can end in a period if you want to make it a sentence.