I moved away from a desktop client for several years because of Thunderbird staying stuck in the 2010s, but the redesign brought me back into the fold. It's certainly overkill for scanning through subject lines, but compared to having five tabs open ...
Mailspring doesn't handle folders well. When I was testing it, it synced my inbox fine, but none of the folders worked. I even set up a dev environment to try and fix it myself, but couldn't get things working properly.
Correct me if I'm wrong, I very well might be, but doesn't Bluemail do the same thing as the new Outlook for their "instant push" feature? I don't see how else they'd accomplish that.
Geary is so close to perfect but they depend on Gnome Online accounts which doesn't support O365 so I can use it for everything but my university email.
I must say I'm quite pleased with it too. The previous time I tried it was in 2005 and it was just ok.
I also recently found out about the Owl add-on. Really makes it a good alternative