Whereas Fastmail charges by the user/month (or did when I was there), which can add up quickly if you've got a bunch of domains, Migadu does not. They charge by volume. You can add as many domains and mailboxes as you like. For some use cases - mine, and likely many of you - that ends up with a way smaller bill.
They're run by real people in Switzerland. I've been a customer for a couple of years now and it's worked flawlessly.
Usual disclaimers, just a happy customer, not affiliated.