This complex question came in yesterday's email:
What is the proper way to address a letter with several addressees in the same letter, but the gender of one or more is unknown? Normally the greeting would be “Dear Misses. So-and-so,” and “Dear Messrs. So-and-so,” but what if the gender of one is unknown? Is it appropriate in that instance to use the general, blanket “Dear Sir or Madame” or “Dear Misses. and Messrs."?
First, let's correct one of the question's incorrect assumptions: There is no Misses that ends with a period, so "Dear Misses. So and so" is never correct.The plural of Mrs. is Mesdames (no period). The plural of Ms. is Mses. (with a period). The plural of Miss is Misses (no period).
Messrs. as the plural of Mr. is correct.
But back to the original question: What should the writer do about the unknown gender and courtesy title? The solution is an Internet search for the information or a call to the individual's office. After all, when you are writing to someone, you must have contact information. Use that information to learn the person's preferred courtesy title.
Incorporating the newfound information, the greeting would look like this:
Dear Messrs. Jones and Washburn and Mses. Gomez and Green:
Or like this:
Dear Mr. Jones, Mr. Washburn, Ms. Gomez, and Ms. Green:
In the first example, Mses. is the best female courtesy title because it addresses the women without regard to their marital status. However, if both Gomez and Green are married and they prefer the courtesy title Mrs., then "Mesdames Gomez and Green" or "Mrs. Gomez and Mrs. Green" is preferable.
If you are writing to one or two people whose gender you do not know for certain, you can skip the courtesy title and use full names, like this:
Dear Dana Marsh and Lynn Williams:
My book Business Writing With Heart answers many more questions about ways to address envelopes and use greetings correctly.
Good luck with your complicated greetings!