There is Franz, there is Rambox (which seems a bit abandoned because of premium version), there are few another I can't remember.

How is your app better than these? I switched from Franz to Rambox because of support of self-hosted and/or custom apps. Is there a reason for another switch? After a quick look - it looks like you don't support self-hosted apps.

And a bit aside of main topic - I really hope everything will get PWA support eventually. It's much better experience to make an app really an app. Still waiting for Google to deliver their apps as PWA, like promised. I could ditch Rambox then.