Prevent uses Spamhaus for blocklist monitoring. Spamhaus is a well-established spam and domain reputation service. The "This email domain is on a block list" error message is usually caused by Spamhaus flagging the domain in question.

You can view the details about why Spamhaus has blocked a domain by navigating to https://check.spamhaus.org/ and entering the domain.

In this situation, we advise contacting the domain owner and alerting them that their domain has been blocked. They can then take the steps Spamhaus has advised to get the domain removed from the blocklist.