> Can this client reply to emails with a custom From address, as catch-all domain users need?

Sadly it does not support either.

we're currently looking towards gmail(current) => outlook => protocol layer for now. However, will definitely keep note of it.

> How does this client compare to the leading open source email client, Thunderbird?

Took a shot at this here: https://github.com/curdinc/skylar-email/issues/61. I'd be glad to elaborate more as needed.

I'd love to know about your thunderbird experience.