cross domain e2e is quirky.
also mail strange itself cannot be e2e encrypted (obviously) so while email body is probably encrypted with password or srh derived from your password (not even sure about that would require client side js in the web client), the metadata is probably still stored unencrypted.
That is the fundamental problem with email as a protocol, yes?
Zero knowledge inbox, externally audited code, a company actually caring for user privacy definitely is a win for me when the alternative is Google hoovering up your data. Is this what you meant by saying it is the next best thing?