grammatical number - What is the proper title abbreviation for addressing multiple people?

I'm writing a letter to several recipients in the same document and want to address them correctly at the start.

Is the following correct (I suspect not) and if not, what should it be:

Dear Mr's Jones, Smith, Bloggs and Flintstone

Answer

The plural form of Mister is Misters, and the abbreviations Mr. and Messrs. respectively (although UK English drops the periods). The odd spelling is because "Messrs." comes from the French "messieurs". So your example would be phrased as:

Dear Messrs. Jones, Smith, Bloggs, and Flintstone

Ms. is a bit more complicated; any of "Mses.", "Mss.", or "Mmes." (from the French "mesdames") are acceptable. If you use one of these, be consistent and don't randomly change to another form.

If it's mixed between two genders, use the appropriate honorific for each set and join them with "and". So for instance:

Dear Ms. Smith and Messrs. Jones, Bloggs, and Flintstone

For addressing a married couple with the same surname, a variant may be employed.

Dear Mr. and Mrs. Smith