We are trying out Skiff Pages for some non-sensitive video production documents because it is one of the few end-to-end encrypted document providers with real-time collaboration support, but I don’t think any of us are using Skiff Mail. We’re also still on the fence with Skiff in general, it’s still pretty new and they’ve made a lot of questionable software decisions (notably everything they’re doing related to Web3) so… Don’t take any of us using it as an endorsement either, we’re still evaluating options.
Skiff Mail does not look too promising at the moment for anything besides collaborating with other Skiff users, the lack of any sort of interoperability in terms of end-to-end encryption is disappointing (compared to say, ProtonMail’s encryption which is compatible with any other PGP service/client). I would probably stick to a more traditional private email provider: