I see a big gap in the market when it comes to self-hosted Office 365 or G Suite alternatives. Microsoft Exchange is a beast both in terms of complexity as well as licensing cost, and I think most small/medium orgs don't actually need the complexity which is why they're going towards G Suite which is a less complex product to manage.

I can see them succeeding with an open-core enterprise e-mail & calendar solution where the base features are free to use and an enterprise version with extra features, like the Gitlab model with self-hosting as the key selling point.