In an email beginning with 'Hello', should one put a vocative comma following the first word? [duplicate]

Great question.

Of late, I've been striving to include the comma. When I fail to do so, I feel as if I have wrongly punctuated the salutation. Still, I suppose it is a matter of context. I'm a lawyer, and most often, my emails are written with formality.

If one is writing an email in a formal context, I believe the comma is appropriate. In an informal context, the comma feels stilted. There is a Chicago Manual of Style Q&A note on this subject, and it suggests the same.

Also, I suppose it is worth making the allusion in your question explicit: In Dear John, the word Dear is an adjective modifying John. In My Friend, the word My is a possessive pronoun modifying Friend. But, in Hello, John, the word Hello is an interjection, not a modifier.