Picking MJML as the output format instead of raw HTML is the move. That's the layer where the cross-client pain has already been solved by people more patient than any of us.

Two questions:

1. How extensible is the block system? Can consumers define their own block types with custom MJML output, or are you limited to the built-ins?

2. Any plans for a headless render path (JSON → MJML → HTML on a server) for transactional emails generated from templates at send time?

Either way, bookmarking. Been waiting for someone to do this without the SaaS tax.