Some more questions, as I don't see them addressed on the site:

1. Is there an extension architecture, that I might be able to write my own extensions?

2. Can I edit the "From" address on outgoing mail, and will it remember my preferred "From" address on a per-recipient basis?

3. Does it have extensive keyboard shortcut support, or does it require a mouse?

4. Does it work well with both of the Linux clipboards, the standard system one and the X-provided middle-click one? (Other Electron apps, such as Postman, do not)