@Tutanota The main selling features I'm looking for are #E2EE interoperability with such competitors as @protonprivacy and @mailbox_org, and a shared non-proprietary API to locally bridge Tuta and competitors with common desktop mail, contacts, and calendar apps.

No one should need to maintain multiple subscriptions or break end-to-end encryption to carry on a three-party email exchange with subscribers to one of the three services each, nor to invite the other two to an event in the calendar. Anyone should be able to view their work email and their private email in the same UI. And many customers will want to bulk drag and drop or cut and paste mail and events from their old Google or Microsoft accounts into their new Tuta, Protoon, or Mailbox accounts.

Make E2EE mail and calendars federated (i.e., protocol-compatible across competing services) and compatible with desktop clients (via a single cross-compatible locally client-hosted bridging server), and you'll remove one the main barriers to customer adoption.

A non-profit trade association (like the W3C, but for E2EE mail, contacts, and calendars) would be the best place for the copyright etc. in the bridging software, and the best employer for the lead maintainers of it. Eliminate the trade-off between vendor lock-in and privacy.

Other features are nice, but till no one needs multiple concurrent competing subscriptions, or multiple mail, contact, or calendar apps, other features are practically irrelevant. What uses is an overview of my personal calendar when I can only see my own schedule there, without the context of my work and university calendars in the same view?