a comma

Hi, dear teachers,
Here’s from jmcnamara@ccc.commnet.edu:
A vocative is always treated as a parenthetical element and is thus set off from the rest of the sentence with a comma or a pair of commas (if it appears within the flow of a sentence).
Excuse me, Mr. Smith, for not recognizing you.
Why do I seem to be the only one to use a comma after Hi and Hello? Is it an exception?

Thanks in advance.

"The comma is often omitted today in direct address, especially in the opening line of an e-mail message or an informal memo. We see "Hi Bill" instead of "Hi, Bill." Read more here, Grammar Gremlins: E-greetings don't need a comma.

David Neely, author of the article Escape from the Grammar Trap, writes, "it's a matter of style. _I_ certainly don't use a comma after "Hello", especially when "Hello" is followed by someone's name, but I don't think using the comma in that situation is incorrect punctuation.

I differentiate between punctuation that is essential or at least very important to convey the intended meaning and punctuation that is a stylistic preference that is not needed to convey the intended meaning. A comma after "Hello" definitely falls in the second category." Source

From How to write E-mail

It is also becoming quite common to write the greeting without a comma,

e.g. Dear Miss Lawson
e.g. Dear KK

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On matters of etiquette, Judith Martin, author of “Miss Manners’ Guide to Excruciatingly Correct Behavior, Freshly Updated” has this to say about the correct way to begin an e-mail. “With almost anything civil. Or even nothing, because an informal e-mail is like a memorandum and doesn’t require a salutation, although an abbreviated one may be used. The Internet is an informal means of communication. But that is presuming that informality is understood not to be a euphemism for rudeness or sloppiness.”

so, what’s the appropriate way to begin a formal e-mail message? “There is no such thing as a formal e-mail, any more than there is a formal telephone call,”...Source