@thunderbird Thunderbird is nonfree, as it recommends and installs proprietary software without even mentioning it's proprietary - just that the ews plugin is "paid".

Meanwhile, Evolution is free software, including ews support (although the built in web browser unfortunately runs proprietary software required for logging into outlook for example, unless you can provide it with the tokens it requires and then it doesn't).

Claws-mail, mutt or rmail are the best choices for POP and IMAP really.