Dear Mr.Donahuue,
Mr. President
Yupp[: Dear Mr. President was written by Pink and Billy Mann.
Write, Dear piano teacher ... or if you know the last name, Dear Mr. (or Mrs.) So and so.
Yes, that's correct.
Dear Dr. & Mrs. Smith
I doubt it, I've certainly never heard or seen it used. If you are writing to a married couple then "Dear John and Mary" (for example) or more formally "Dear Mr and Mrs" are the usual forms of address. For other pairs of people their first name or more formally their surnames would be correct. For example "Dear Martin and Simon," or "Dear Mr Smith and Mr Hughes."
Date Mr. Harold Happy, Jr. street address city, st zip Dear Harold, OR Date Mr. Harold Happy, Jr. street address city, st zip Dear Mr. Happy, - - -
Dear Mr. Henshaw was published in 1983.
Dear Mr. Capote was created in 1983.
Dear Mr. Wonderful was created in 1982.
Dear Mr. Prohack was created in 1949.
Cher Monsieur is Dear Sir. if you know his name is Dupont, write Cher M. Dupont