Dear Sirs,
men
Gentlemen:
Gentlemen and Madams
Gentlemen, ...
In the salutation line of a letter addressing multiple men and women, you can use "Dear All" or "Dear Team" for a neutral and inclusive approach. If you prefer to be more formal, you can list the recipients, such as "Dear Mr. Smith and Ms. Johnson," or simply "Dear Mr. Smith, Ms. Johnson, and Team." If there are many recipients, "Dear Colleagues" can also be appropriate.
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You put in your address and instead of saying "Dear sir," you would say, "To whom it may concern"
You could write "Dear Sirs and Madam" in a business letter, or you could write "Dear Friends" in an informal letter.
Messers is just plain incorrect in any language; Messrs is the abbreviation (pronounced "messers") for messieurs and is used in English; Messieurs is French; Messrs or Messieurs is a term used to address many men rather than "Mr Pink, Mr White, et al."
To address a letter to several people, you would include each recipient's name and address in the address block. The salutation would be "Dear Sirs and Madams" if you are not familiar with the recipients. If you are on familiar terms, you would address as "Dear Jack and Jill." If you are addressing a letter to a company or entity and you are not addressing a particular individual, you would use "To whom it may concern." That salutation is inappropriate when you are addressing particular individuals.
Messrs. Smith and Jones Messrs. Smith Messieurs Smith and Jones Messieurs Smith
No. Most men do not have multiple orgasms (pretty sure they would if they knew how. . .)