[quote="JimG, post:14, topic:269826"]
In the olden days at the office where I worked, business letters often began with “Gentlemen:” no matter who it was addressed to. Then we went to “Dear Sir,” unless the recipient was known to be female, in which case it was “Dear Ma’am.”
Later we went to: “Mr. or Ms.”
And ended up, last I knew, with “Ladies and Gentlemen” which sounds rather like the opening pitch of a circus barker.
Since "Dear Sirs" is correct, I'd use "Dear Sirs and Madams" or "Dear Sirs/Madams" or "Dear Sirs or Madams" and definitely "Dear Madams" for more than one lady (and no man). "Ladies" only when acquainted enough to pull it off. I like that use of "Gentlemen:" but I'd probably go for "Dear Sirs" anyway (or "Messrs" is specific circumstances).
When I don't know who it is, and I'll go to great lengths to find out (e.g. searching people with photos on Facebook who have the same first name), I'd typically go for "Dear Sir/Madam" to avoid the more potentially offensive "Mr/Mrs" or "Mr(s)".
This reminded me of something. I remember using "Homies" on purpose in a law firm. It was a gag by junior lawyers. None of us works there any more but we still retain this habit.