Which honorific is used to address a married woman in English-speaking countries?

"Mistress" used to be a very honourable title, but now it has changed its meaning.

TRUE CANADIAN Laurel, Calm down, mellow, no reason for this. As the explanation stated American English or British English (which would be, as you stated, The Queen's English) is equally represented & acknowledged here. Yes, there are many places in the universe that English is spoken, however, the Q/A/E with this specifically singled out the two as American English & British English.

Ros, With the growth in " gender fluidity" their are an increasing number of options. I can see it getting down to "M". After all, what does it mattet? Especially for business purposes.

Well, since men don't change their " honorific" ( though I dont see much honour in it) when they marry, women shouldnt need to. In France, the title is far more to do with age. All older women are called "Madame" whether married or not. Though they may feel flattered if someone says "Mam'zelle "!

little b, Yeah me too! I hope so! and will be I know it!

A little outdated. I live in an English speaking country, am married and I’m a Ms.

TRUE CANADIAN Laurel, Canada is in America, North America

Mrs derived from the contraction Mr's, i.e. belongs to Mr.