mailbox.org and posteo.de are both 1€ per month. I haven't used either though.

I use Posteo and would highly recommend it. There may be occasional hiccups, but those seem to be few and far between.

This 1-XS server costs 2€ every month and it could perfectly handle the email of hundreds of users. They are charging you more only for yourself, and that's not even factoring in the economies of scale.

https://www.scaleway.com/virtual-cloud-servers/#anchor_start...

Fastmail has employees that have to be paid. You're paying that money instead of the time required to maintain your own email server.

I understand that, but paying 100x what a VPS would cost, that's a rip-off. (100x because a VPS can handle the email of 100 people or even more.)

But what does the price of a VPS have to do with anything? I doubt hosting would be their biggest cost.

Why shouldn't hosting be their biggest cost?

Well, think about the VPS you're proposing. Two euros a month is 24€ a year. Even if you're only paying yourself 6€ an hour, I'm skeptical that running an email server for 100 people would require less than four hours per year.