Where should the comma be placed in the salutation of a letter?

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?