This sounds like it could be potentially useful, as an alternative to unopinionated productivity tools which force you to make far too many decisions upfront.

This sounds like it could be potentially useful, as an alternative to unopinionated productivity tools which force you to make far too many decisions upfront. On the other hand I've never heard of Kanban and I'm not the only one. I feel like the name alone is intended to communicate what this does to a core part of your target audience, but if you've never heard of Kanban explaining that it's a "Kanban board for your emails" in the tagline doesn't make it that much clearer either. Either way, good luck with it! I'm a screen reader user so would be interested to see how accessible and usable it is from the keyboard alone. That's also why you should read what I've said above in the context of not being able to see your visual design, which might instantly make it clear what it does.