Ask HN: Free Email Hosting for Nonprofits?

Hack Club is a nonprofit network of hackathons, student-led coding clubs, and open source projects. Our website is https://hackclub.com and our GitHub is https://github.com/hackclub.

We have been receiving free email hosting from Google Workspace and providing it to the Hack Club network, but we recently hit the domain limit (600 domains) on Google Workspace for Nonprofits. Each domain is typically a hackathon or a chapter at a high school.

Does anyone have any recommendations for email hosts that we could look into? As a mostly volunteer-driven nonprofit, we can't afford pay per-user pricing as there are thousands and thousands of accounts.