ISP's spam filters use blacklist filters to identify and block emails from known spammers.
They compare the email's IP address and domain name to those on existing blacklists. If the email matches an entry on the blacklist, it will be marked as spam and either blocked or sent to the user's spam folder.
To avoid being blacklisted, email senders should regularly check their IP
addresses and domain names against existing blacklists using specialized tools.
If they are listed, they should address the issues that caused them to be blacklisted, such as identifying and removing compromised accounts or addressing email deliverability issues.