I think we have to give credit where it’s due and a lot of Linux apps (mostly the non-distro ones, so no Gnome apps etc.) Do have quite dated interfaces. I can’t find a nice mail client except mailspring and bluemail, both of which then come with some features not working (depending on your distro etc.)
Here is a screen-shot of latest Thunderbird (ESR) Email client: https://www.thunderbird.net/media/img/thunderbird/new/screens/mail-screen.png