Thunderbird is good, but I'll also suggest Postbox for people looking for alternatives. It's a commercial fork of Thunderbird (so about as "native" as Thunderbird is), but I find Postbox has a nicer UI, and at the time I switched its local email search was significantly faster. Layout is customizable, you're not restricted to the design (or theme) in their screenshots:
https://www.postbox-inc.com/
It has some support for Thunderbird plugins. I use a self-modified version of h.ogi's Priority Switcher to bring back the user-editable priority column that Eudora used to have. (I don't understand how people can process their mail without sorting it by Priority....)
Won't be for everyone - it's not open source, not free and there's no Linux version (yet?). Personally I'm happy to pay for a professional / premium email client experience, since it's so mission-critical to my work.
I'm also a Postbox user and quite happy so far but I don't know if I would recommend it. Development seems to have stopped and Thunderbird is making good progress on the redesign.
Won't be for everyone - it's not open source, not free and there's no Linux version (yet?). Personally I'm happy to pay for a professional / premium email client experience, since it's so mission-critical to my work.