For some reason I always thought that Geary https://wiki.gnome.org/Apps/Geary was some weird email client which needed some special server from some company, and I never wanted to send my emails to a different server to be able to use a desktop client. But I think I'm remembering a different client.

Geary seems to be part of GNOME and it's nicely integrated there. It starts extremely fast compared to Thunderbird and the dark theme was automatically chosen according to my GNOME settings. And even pulling down the emails and the UI are super snappy. I think I need to try it out for some time.