Didn't really respond to my question.
Hello. We integrate third party tools SpamAssassin, SpamExperts, and Box Trapper to assist customers with fighting spam. You'd need to contact the respective organizations regarding their methods for marking specific domains and origin IP addresses as spam. Much of it is based on known malicious patterns and indicators.
You can also contact our Live Support for assistance figuring out why a domain was blacklisted and how to whitelist it.
If you're trying to keep your domain out of spam boxes, follow our article on how to block spam and protect your emails from being marked as spam.
Again, if you have questions regarding something blocking specific domains, you should contact Live Support directly with relevant information to fully understand your issues.
Customer support service by UserEcho