For the moment, it's just one person (myself). I've been working on this project for the last year. I started a company in order to be able to receive donations and hopefully raise some money from YCombinator or VCs.
My long term plan is to make an open source alternative to Google Workspace (not just e-mails and calendars but also Documents, Spreadsheets, etc) but I won't be able to do all that by myself so that is why I'll start looking for funding in the near future.
Have you considered going the donations / sponsorship (e.g. patreon) route. This seems like it leads to much more sustainable open source projects than VC money which often leads to a project flaming out when the investors come to collect (or the company gets acquired).
As a data point (probably closer to anecdote), I'd be willing to pay for just an email client. I don't know about recurring payments yet, but something like Superhuman-but-cheaper is definitely something I'd consider. My only real option right now is Outlook, I think.
My long term plan is to make an open source alternative to Google Workspace (not just e-mails and calendars but also Documents, Spreadsheets, etc) but I won't be able to do all that by myself so that is why I'll start looking for funding in the near future.