Claws and Sylpheed come to mind.
* https://sylpheed.sraoss.jp/en/download.html
Compare those to https://www.thunderbird.net/en-US/thunderbird/all
I can't find a current download for modern macOS on either of the first two
Really shouldn't need to explain that, IMO.
It runs everywhere and can be installed by anyone.
Why do want to make it harder for people?
If that’s not gatekeeping I don’t know what to tell you.