All are used. Using a full stop/period after Mr and Mrs makes the style look decidedly dated in British English but not using one in American English would be very uncommon. The Brits stopped placing a comma after Smith long ago: Dear Mr Smith CB
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English 1b3What are your thoughts on the semicolon?There are no semicolons in your original post.
Cool BreezeA comma after Dear is impossible and after Hi it is used less and less frequently.hi,
Cool BreezeThere are no semicolons in your original post.I forgot to add this one. I just remember reading an example where the semicolon was used.