Spamhaus is the world leader in supplying realtime highly accurate list of domains and IP that have a bad reputations of sending out malicious content. They do their own research for any suspicious activity and reports of domains and IP address then use their criteria to mark them on their Spamhaus Block List (SBL). Most email providing companies use their SBL.
We use spamhaus to help prevent any potential spam or phishing when sending and receiving emails through our mail server. If any email is detected to be associated with the SBL it would cause the email to be return to the sender as a bounce back email that will inform the sender that the email was rejected per being on the SBL.
To have your domain name and/or IP address removed from the SBL you will need to submit a request to Spamhaus. This link will take you to their site that you can request the removal. They will reply back to you as soon as possible with a update.