Although it does not really answer your question,
https://www.fsf.org/resources/webmail-systems lists "Free Software Webmail
Systems" that usually make privacy promises too.
- Re: [Trisquel-users] An alternative to gmail fredo
- [Trisquel-users] Re : An alternative to gmail lcerf
- [Trisquel-users] Re : An alternative to gmail pinmaritim
- [Trisquel-users] Re : An alternative to gmail pinmaritim
- Re: [Trisquel-users] An alternative to gmail t8mf4nu6lizp
- Re: [Trisquel-users] An alternative to gmail john_harvey
- Re: [Trisquel-users] An alternative to gmail fenderbassist