OpenPGP
As far as I was able to find out, Proton is the only web-mail service that is fully interoperable with OpenPGP {https://www.openpgp.org/). (It does not implement W-O-T, but so does not - for instance - Thunderbird native OpenPGP).
Can somebody please correct me if I failed to identify another service?
Implementing "E2E mail encryption" only for the traffic between users of the same web-mail service vendor is pointless, even if done "lege artis" - which this is far from.
Biting the hand that feeds IT
