The E-mail Firewall Service subscribes to the Spamhaus Block List (SBL) filter system of spam-blocking. The SBL is a continuously monitored database of IP addresses that determines whether or not e-mail is being sent by a verified spam source. For more information, see: http://www.spamhaus.org
Can I check myself to see if the sender is on the SBL list?
Do you have an e-mail message that you received previously from this person, organization, or e-mail list? You can check yourself to see if the IP address of the mail sender has been blocked.
How will the sender be notified?
Senders of known spam that are blocked by the spam-blocking system will see bounced messages, for example:
550 5.7.1 Email rejected because 127.0.0.2 is listed by sbl-xbl.spamhaus.org
554 5.0.0 Service unavailable
The message means that the IP Address "127.0.0.2" is listed within their database of blocked addresses.