Sometimes I see a comma after the proper name:
Hello Mr. Black,
In order to give you....
But my native language is not English and I think that the comma in this phrase should be placed before the proper name:
Hello, Mr. Black.
In order to give you....
What is the correct punctuation in English?
Both are correct.
In a dialogue, a pause can be used for effect, putting emphasis on the greeting, eg:
Hello, Mr. Black.
"Hello" is said first, then a pause, then the name. Normally, people would say:
Hello Mr. Black.
Without any pauses in their speech.
When writing a letter or email, it is quite common to do this:
Hello Mr. Black,
Bla bla bla...
Or in a more formal situation:
Dear Mr. Black,
Bla bla bla...
It is uncommon to put a comma before the title even in an informal email salutation (though quite acceptable), but it would never be done in a formal letter/email.
Not placing a comma before a proper noun will change the object of the sentence. These examples should explain:
Let's eat, John! (Correct)
Let's eat John! (This means you'd like to eat John.)
Let's eat, everybody. (Correct)
Let's eat everybody (This means you'd like to eat everybody.)
When I read
Hello, Mr. Black
I find that I mentally need there to be a comma at the end as well, or perhaps a full stop.
Hello, Mr. Black, how are you today?
Hello, Mr. Black.
And the result is that the name is emphasized. Whether you want to do this or not depends on the circumstances, but for letter writing this is usually not what you want. If you simply want a standard greeting put the comma after the name (or a full stop if the greeting is your whole sentence).
Hello Mr. Black.
Hello Mr. Black, how are you today?
In formal writing you would put a comma after greetings: "Hello, Mark." In emails and informal writing, it doesn't make a whole lot of difference. I, for one, feel self-conscious about using commas like that in emails; it seems too formal.
In the specific case of the opening salutation I would not insert a comma between the greeting and the name if a comma came after the name, but I would if some other punctuation followed the name.
The following examples all "look correct" to me:
"Hi Mark, how are you?"
"Hi, Mark. How are you?"
"Hello Mark, what's up?"
"Hello, Mark! What's up?"
But the "Hi, Mark, how have you been?" example that you gave looks over-punctuated to me.
The title of the question said “in a letter” in which case you would be better sticking to:
Dear Mr. Black,
How are you today? (The the rest of the letter follows.)
Other forms may be grammatically correct and certainly not a problem in an email. But for a the more traditional context of a letter, I think your safest bet is this one.
You need to place a comma between the salutation and the name of the person addressed. "Hello, Mr. Black. How are you today?"
