Salutation and Punctuation

[Anglophile]

When we address the recipient of a communication, which of the following salutations are correct?

Dear Madam or Dear madam
Dear Friend or Dear friend
Dear President or Dear president or Dear Mr President

Further, is it necessary to punctuate it with a comma in an open-end letter format?
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Anglophile
I know only one thing - that is that I know nothing.

[Risna]

I think in formal letter, it is used colon Dear Madam:

[Anglophile]

I doubt if the colon is suitable after the salutation.
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Anglophile
I know only one thing - that is that I know nothing.

[Alan]

Dear Madam /Dear Friend /Dear (Mr) President - those would be my choices followed by a comma and the first letter of the word in the next line would be a capital letter.
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English as a Second Language
You can read my ESL story Present Simple

[Anglophile]

Thank you, Alan. I also prefer initial capital letter for the word that follows 'Dear'.

However, in an open heading, I see no punctuation after salutation, after subscription, after date etc as indicated below.

XYZ Pvt Ltd
Downing Street
London

Dear Sir

Yours sincerely

November 14, 2017
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Anglophile
I know only one thing - that is that I know nothing.