I’m visiting the site of Purelymail from time to time for over a year now (shortly after they launched), because I’m interested into how the service evolves. It looks like a great service that provides purely mail and is very cheap or even cheaper when you are opting for advanced pricing (“Pay as you go”). You can add as many custom domains and users as you want and just pay for the resources you actually use.
Unfortunately, they are still in beta and not yet “GDPR compatible” (I didn’t find anything about an option to sign a DPA). I would also like to read some feedback from existing users.