Some email addresses can't be used coz they falsely decided my mailserver is a spam source.
Considering how few email messages are sent out, and they are only sent to people for account notifications, it's really just a royal PITA.
So if the web page says you can't use an email address, it will be because the mailserver you want says I'm a spam source.
That's a shame. Have you tried submitting a retraction to all of the regular RBLs?
So many times ... I've been running email servers for over 25 years.
Some of them also have no way to be retracted.