And I can't find pricing, TOS, talk about data access (from the features, it appears they want to pull my mail through their servers?) - and a few features that breaks with good email/netiquette (read notifications, links in email that rely on http(s)).
What is this product? What's the monetization plan?
If they're only targeting business users... why don't they also support Edge? Weird. Doubly weird that all their screenshots are in MacOS, which has plenty of safari users, and isn't enterprise-friendly.
So they really are carving out a very small niche - small upmarket businesses that are willing to pay for something other than gmail (when paid gmail also gets you the rest of the google apps suite). Feels like a shonky business-case to me - I can't see how they'd competitively price the accounts if they're basically ruling out companies with large user-bases.
Now, what kind of email servers/hosts do they support? Exchange? Only Gmail? Any Imap? Pop3? Do they have a policy on tls support?
As for market/product/niche - I actually think carving a thin slice out of the business market is a great idea. You could probably get target/interested businesses to pay a pretty premium for a "good email solution", whatever that means for any particular business. I don't think neither exchange nor Gmail would be particularly hard to compete with as a product. Much like some will pay a pretty premium for an ide that boost productivity and/or happiness...
That said, I'm now pretty sure I'm not a target customer for superhuman. But that's fine, I whish them best of luck.