I took a look at Mailspring, but it was pretty much a showstopper when I couldn't create an account for a user with a single name.
In my case, the email account that I use the most is one for a user called WetGeek. No last name. (I'm a retired software engineer in a part of the US where it is said to rain all the time.) Thunderbird has never had an issue with that. And when I tried to use Wet Geek, of course that didn't work, as the mail server for my domain doesn't recognize Wet Geek as a valid user name. I couldn't try it with my gmail.com account, for the same reason.
Seems like a decent mail client with some potentially useful features, but I couldn't get past the setup routine. Maybe that restriction will be removed at some later date, and I can take another look at it then.