For a standalone, Thunderbird. As the old ads said:
“What’s the word? Thunderbird.
What’s the price? Mighty nice.”
Or get the Seamonkey suite - it has an email client built in.
https://www.seamonkey-project.org/start/
The envelope icon at the bottom left launches the email client; it also has a non-Google based browswer, a basic HTML editor and some other features built in. If you want to avoid the Google/Microsoft to the extent possible on a Windows system, it’s the way to go.
SeaMonkey, from what I remember about it, resembles the old Netscape Navigator and that, for old timers like me, is also a great selling point. For anyone looking for an all in one package, it’s definitely worth looking at.