Punctuation in Formal Letters and E-mails

The question that I have might sound slightly silly but here it is: Is a comma required after the salutation in a formal letter? I currently don't use one but I have seen formal letters being written with and without the comma after the salutation.

1. Hello Mr. Smith,
2. Hello Mr. Smith

So, which of the above two is correct?

Thanks in advance for the help :)

[Tdol]

I would write:
Dear Mr Smith,

[Laidbackness]

Hi, Thanks for the reply. I understand that the use of comma after salutation is common and therefore probably more acceptable as well. I was looking at an online guide yesterday and they mentioned the concept of Open and Closed Punctuation in formal correspondence. Open Punctuation does not require a comma to follow salutation and closing whereas in a Closed Punctuation comma is necessary at both the places

That brings me to the next question. What about the use of colon (:) after salutation?

Example, Dear Mr. Smith:

If I use a colon, then do I need a comma after the closing?

1. Yours sincerely,
2. Yours sincerely


Which is correct?

[Tdol]

The colon is American usage and this site suggests that when you use it, a comma should be used: Letters