"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.
Hi, dear teachers,
Here’s from EMAIL REMOVED - Send PM to This User Instead:
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.
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
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