[m-p-3]

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.

[Vinnl]

For an example of that, see here: https://github.com/wesbos/burner-email-providers/pull/339

But yes, definitely a concern that is constantly on our radar.