Migadu is a small, independently-owned, e-mail provider based in Switzerland.
You can create as many accounts as you want for as many domain names as you want, but you don't have to pay for each account separately. Instead, you can purchase a subscription plan that allows for X emails received per day and Y emails sent per day. And all the e-mail accounts under your plan share those daily credits. One e-mail account can monopolize all the credits in a day, so you need to make sure that the plan you select can accommodate all your e-mail accounts.
It's a really neat service. I've used it for years and I have no complaints.
Prices start start at $19 per year. They accept credit cards, Paypal and Bitcoin (by special request).