Just sent an email with a very polite explanation about why it is inappropriate to address a professor (or honestly, anyone unless they tell you to) as "Mrs."
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@cfiesler when I was in grad school the culture was that everyone was on a first name basis. One professor came to class one day and asked that she be addressed by title. A student asked if she would give her students the same courtesy. It turned out that there were 2 PhDs, a JD 2 MDs, a judge, a state senator, and a police officer who all would have had professional titles. We went back to first names. Ironically the student who made the fuss is now entitled to be addressed as "The Rt. Rev." or "Bishop"
@antares I'd be happy to call my students by their titles. And would never call any of them Mrs. unless they'd explicitly told me they were married and that's how they prefer to be addressed.