From today the mail server is unable to receive any more messages as they are all considered spam. I tried deactivating the anti-spam filter and regenerating the email domains but the problem remains. This is the message the sender receives.
SMTP error from remote server for RCPT TO command, host: mail.mydomain.it (159.xx.xx.xx) reason: 550-Rejected because 212.xx.xx.xx is in a black list at zen.spamhaus.org
550 Error: open resolver; https://check.spamhaus.org/returnc/pub/2a01:4f8:0/
Why has my email not been delivered?
The problem is with the recipient’s email server configuration.
This is not due to an issue with your email set-up.
It is not because you are listed on one of our blocklists.