You can flag them to the Relay team and AFAIK they'll reach out to the domain that blacklisted them with the hope to make them change their mind.

A service that doesn't accept an email proxy during registration is not going to respect my privacy, so IMO not worth of using.

Sites already have started blocking the mozmail.com domain name. I'm considering moving over to fastmail given that their domain is already established for email purposes. I would imagine the same is true for icloud emails