All I can suggest is the FSF's list: https://www.fsf.org/resources/webmail-systems
I currently have a Riseup.net account, but their canary died a few months ago, so I would recommend that people do not create new accounts there, and anyone who does have an account there should take steps to prepare to leave it for another service. The ones I'm looking at are Safe-mail.net and VFEmail.
Of course, though, the best option is always to host your own email, if you can.
