Advice: Use “Professor” or “Dr.” in email salutations and do not refer to your female professors as “Mrs.” unless they have requested that you do so.
Why: “Professor” or “Dr” are the appropriate honorifics for all faculty members, male and female, unless instructed otherwise. This matters to many faculty members for the same reason that we all prefer to be identified by our actual name, not a false name. “Professor” or “Dr” accurately indicates the professional identity of a faculty member. Furthermore, “Mrs.” refers to a marital status that may or may not apply to your female faculty members; you should not presume that it does.