This error message is generated by our email provider. They also maintain a suppression list, which can lead to some weird issues.
In this case, your email address was blocked because it returned this error at some point:
550: 550 Requested action not taken: mailbox unavailable
I’ve removed your address from the block list at our email provider. If you have confirmed your email address works, you can click the “I HAVE FIXED THIS” button to get a new verification mail.