[janc_]

Most RBLs are scams. No competent mail admin uses them to block mail ever.

[rufo]

…lots of major mail services do?

https://www.fastmail.help/hc/en-us/articles/360060591413-Spa...

https://senders.yahooinc.com/smtp-error-codes/

https://www.xfinity.com/support/articles/email-errors