Complete stranger? There are formulas like “to whom it may concern”, “Dear Colleague”, or (best?) just “Dear [full name]”… As likely as not, you will not be able to guess whether the person is a man or a woman, nor whether the person is African-American, and even if you could-- say you knew the complete stranger was a male Bhutanese-Indian-Samoan-American-- would you be less polite than to anyone else?
I’ve used the “Dear Pat Smith” salutation, since it avoids guessing at whether the person is male, female or non-binary. Plus there are various cultures in which the family name is the first one.
That’s a level of abstraction removed. I’d use those only of I had no idea who was going to read the message. If it’s a message for a particular person I’m not going to do that. I offers the message “I have no idea who you are and I haven’t bothered to find out.” When in fact the very reason I’m communicating is that I know who that person is, but we haven’t been acquainted.
It’s an option, but it’s not necessarily best. Usually you get a salutation like that from a corporate or governmental entity, not from a specific person. Again it’s a level of abstraction.
There are circumstances when you might not be able to guess, but I disagree that it’s “more likely than not.” And most of the time it’s not the case that you’ll have zero information about that person.
I’ve spent my professional career contacting people who have no idea who I am, but I have some idea who they are and it’s the reason I’m writing.
It wouldn’t be necessary to guess, and I don’t know why you think I would be less polite to anyone based on my guess at es ethnicity.
I certainly didn’t imply that. You think I’m sitting there trying to puzzle out people’s ethnicities? I’m not.
But being aware of this feature if Afeican-American culture reinforces the practice of making first contact with “Dear Ms. Smith” for everyone, whether or not I know es ethnicity.
If the person prefers “Mrs.” or something else, E will find a way to communicate that.
There are no foolproof situations. If you are in a situation that is absolutely unusual, then there might be a slight awkwardness, but usually the attempt to follow common etiquette is successful.
Of course things are always in flux and one must remain aware. But as of now there is still a hierarchy of politeness, and the current situation is that “Dear Ms. Smith” is a safer initial salutation than “Dear Jane Smith,” which makes you sound like a database. In most cases, things will quickly slip to “Jane.” “Smith” is rarely used as a a form of address in professional situations.
All of this can change of course. In some years it might be considered quite rude to initiate contact with “Dear Ms. Smith” and “Dear Jane Smith” might become the rule, but as of now it’s not the safer bet.
Anything can happen. When I was a kid it was quite rude to address any person who was elder or senior to you by es given name alone. Everyone had a title of some kind—Mom, Aunt Jane, Ms. Smith. And I still find it slightly odd if I think about it to address my superiors at work as “Jane,” but people would think it strange If I didn’t. That’s how it goes. Society and etiquette change pretty
Stupid old joke from the 1970s: "Why not change the titles for men? You can have Mr. for “missed her” and Gr. for “got her”.
I don’t know about you but before I contact someone out of the blue I do some minimal research. For example, how did I know that I wanted to contact em in the first place? Very often from someone else. What information did that person give me? Does E have a LinkedIn, or a profile page on es organization’s website, or a personal website? Any reputation at all?
As for cultures with different name ordering, well, obviously. It’s why I use the terms “given name” and “family name” as opposed to the Anglocentric terms “first name” and “last name.” I stay aware of the cultural possibilities. If I’m writing to someone in Japan or Korea or China or Hungary, I’ll keep that in mind. I don’t rely on a blanket rule so that I can remain oblivious.
And, yes, given all that, one still might make a mistake. But you know what? Mistakes are usually okay. Everyone makes mistakes. It’s what you do upon realizing your mistake that matters.
People get my name and gender wrong all the time, because they’re unfamiliar with my cultural background. They don’t know that it’s a masculine name in my culture and to them it’s either feminine or looks kind of feminine. If they see my face thats a correction right there.
It’s not a big deal. Sometimes I have to find a nice way to correct em. Most of the time I don’t, because E will realize es mistake emself. Or the interaction is so brief that I don’t really care if E ever gets corrected.
Interesting thread.
As a child of the 80s I was also taught to use “Mrs.” if you know she is married and “Ms.” if you know she is single or do not know her marital status.
The idea of using “Ms.” even if you know she is married seems strange, but I can go with it.
I graduated HS in 1981 and I was never taught that. Ms was specifically designed to be agnostic about marital status. What you describe is diametrically opposed to the original meaning.
I went to high school a little earlier and it was no different. I was also taught to address people by the title they prefer. It was also perfectly allowable to address a woman of unknown marital status as Miss. Also Mrs. was often reduced to Miz in some dialects, which is not the same as Ms. And Miz is oddly the name of a male wrestler, while the current women’s champion calls herself The Man. So don’t blame people for being confused by all this.
Good [evening/morning/afternoon] Firstname;
Brisk, formal enough, and right to the point.
It may be formal enough for you- but there are many situations in which it is not formal enough. There really isn’t a genderless alternative except for Mx. or maybe M___ , but I’m not at all sure most people will understand those.
I graduated high school in 1978, and by that time, in my world, “Ms.” was preferred over “Miss” and “Mrs.” in all situations.
Back in the 70s this was all in a state of flux. I had a boss for a while who liked to be called ‘Sir’, so I just stopped calling him anything - “Excuse me…” “Good morning…”. His boss was happy to be addressed by his first name. I got married in the early 80s and my wife and I discussed it - she wanted the world and his wife to know that she was married: she wore a ring and made it clear that she was not ever a ‘Ms’.
On emails (who writes letters these days?) I write “Dear Jane Smith”, the first time, and then copy however she signs herself - usually ‘Jane’. I get the impression that those of you from North America worry more about this kind of thing than we do on this side of the Atlantic, but I may be wrong.
That would be a situation where non-Hispanics would call me something which is very much not my name. And unless we’ve already been introduced, you have no right to call me by firstname; doing so when you’re trying to sell me something in a cold call would be a surefire way to avoid that sale.
I was taught Ms. as a marriage-status-neutral form by the Brother who taught us ESL between 6th and 8th grade, so in the early 1980s.
I work as a paralegal, and in our office all correspondence to any female (email, letters, etc.) begins with “Ms.” Since we don’t know most of these people personally, it’s the best way to handle the salutation.
After the first usage, it can be abbreviated to “the Honorable Clinton.” This avoids confusing her with her husband. 
Yes, I never heard of Ms. being the preferred title mainly for divorced women, but I could see the logic why many would prefer that honorific for themselves.
If you get to the point where neither Mr. nor Ms. is acceptable, then why bother making up an honorific? Ms. was deliberately designed to eliminate the need for Miss and Mrs.; if it too is obsolete, then so are any titles. I presume the majority of people who change their gender identities want to be known by their new one, not hide which one they now identify as.
Oh, and “they/them” is perfectly acceptable English in situations of ambiguity, no need to make up new pronouns either. If you are not sure a person (singular) has a specific identity, they can be called “they” in third person written or spoken English and have been for a century or many. “Whichever pupil turned in this essay they did a good job!” or “The driver then turned on their windshield wipers.”
Just be glad we’re not having this discussion in French or Spanish or Italian, where something resembling gender is baked into every noun, pronoun, and adjective.
or perhaps where necessary use masculine to mean both - one of my old fashioned English teachers expounded the rule “In this case, and wherever desirable, the male embraces the female.”
What’s wrong with “Whichever pupil turned in this essay did a good job”? The pronoun seems unnecessary.
Surely you can remember when you were in school. Generally students still call teachers by their title: Mr. Smith, Ms. Wormwood, etc.–at least to their faces. 
Tsk. His name is The Miz. Leaving out his gender-neutral definite article is just rude.
Actually, it is exactly the same. The term Ms. was coined by simply respelling Southern U.S. “Miz,” which already covered both Miss & Mrs.