Ours servers use spam filters based on these DNSBL/RBL:
Spamhaus - https://www.spamhaus.org/
Sorbs - http://www.sorbs.net/
SpamCannibal - http://www.spamcannibal.org/cannibal.cgi
If someone sent you an email from an IP address that results in one of those RBL, ours servers will not deliver the mail and will send to the sender a message like this:
The IP Address of the sender (184.108.40.206) was found in a DNS blacklist database and was therefore refused
Who's trying to send you the email and receives this message should contact his internet provider and ask him to remove his IP address from the blaklist/s.
Ours servers will accept the email when the IP will not be in those DNSBL lists..
Do you want to know if an address is in a blacklist? Just insert here the address and click "Blacklist Check".