The American standards for a letter salutation:
personal letter: Dear ________, (please note the comma)British standards may differ from the above.
business letter: Dear Mr. (or Ms.) _________: (please note the colon)
Email may use either the comma or colon as above; the colon setting the tone for a more formal address.
I know of no salutation standard for either email or letter that uses the semicolon.
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