LOL! "Mx!" Love it!
I was taught by my mom that you address a married woman as "Mrs Husband's Name/Initial Surname" and a divorcee as "Mrs Her Name/Initial Surname" but that a widow had a choice. Of course, all that has changed now. Perhaps the formal rules are still the same, but I don't know anyone who replaces a wife's name with her husband's, except maybe in the case of much older women who were used to that.
When I was in high school, my friends and I would often scribble imagined married names in our notebooks, depending on who we had a crush on. For example, "Mrs Charles Smith." However, by the time I was half-way through college, most of the girls I knew were saying things like, "No, I would never be Mrs Guy's Name Smith! I'll be Mrs/Ms Annie Smith" or sometimes just "Annie Smith," etc.
I still know a few women, however, who take pride in "wearing" their DH's name.
DD didn't change her name when she got married. So when writing to her, I just address it to Ms. (yes I use that - convenient) Susan (not her real name) Bloggs. When writing to both of them, I send it to "Bloggs and Smith." And when to them and their kids, "Bloggs and Smith" on the first line and then "and family" on the second.