The Vivaldi browser has an email client built in.

Nicely old-school for us old fart who don't care for desktop applications looking like web pages.

So does Seamonkey, and it has the upside of looking absurdly retro too.

Fsck, yeah.

seamonkey feels to much like win 98.... i'd prefer a softer toned, rounded ui for my periodacal (yikes) glances through the mail

+1 for Thunderbird, Switched back to it from desktop Outlook a few years ago and never looked back.

As for the "Supernova" changes, they only affect new installations by default, and you can revert the UI changes with a few clicks.
https://support.mozilla.org/en-US/kb/getting-started-thunderbird-main-window-supernova

downloaded terrorbird and realized that having a choice to remove the office workbar in windows outlook 2023 would a be a step in the right direction.....