Yes you should use a capital letter.
yes.
Yes, when addressing someone directly in a letter or email, you should typically use a comma after the salutation. For example, "Dear Sir," or "Dear Mr. Smith,".
Formal salutations are always followed by a colon. (Dear Sir Only in informatal writing do you use a comma. (Dear Sue,)
A comma is not required in the salutation of an editorial letter. You can use either a comma or a colon after the recipient's name. For example, "Dear Editor," or "Dear Editor:".
No. After the word and comma can not be used, as it is grammatically inappropriate to use comma after conjunctions such as and, which are called coordinate conjunctions.
In business letters, you should use a colon (:) if you use any punctuation. The new way is to have no punctuation after the greeting in business letters. But if you omit punctuation there, you also have to omit the comma after the closing. In personal letters, people use a comma after the greeting.
"Dear Sir," is all you need to use.
Madam is appropriate only when you know the recipient is a female but her name is not known.If you know that the recipient is a woman and you know her last name, use the term Ms. The marital status of the recipient is not appropriate and is irrelevant.If you are making a generic response when you are not sure who will receive it, then you Dear Sir/Madam or Dear Sir or Madam.
If you know their name, then use sincerely. If you don't then use faithfully. Thus Dear Mr Bloggs - Yours Sincerely Dear Sir - Yours Faithfully
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No, not usually but some emails have to have every letter and digit correct.