Tuta previously known as Tutanota is an encrypted webmail app that also has a desktop client. It works on any OS and there’s also a mobile version.
A free account is limited to 1GB storage while other premium plans are available with 20GB storage and 500GB.
If the recipient has the Signal app or another encrypted app, but isn’t a Tuta subscriber encrypted email can still be sent by sending the encrypted password via the mobile app. Otherwise, it’s also possible to send unencrypted emails.
I have a premium account there myself with data synced to the Tuta app on my phone.
More info at Tuta.com
