Collaborate to share preferences with other mail receiving sites.
You decide which networks to allow - with your users and the rest of the quorum community. Your preferences inform the rest of the community.
Once, e-mail was reliable. Now, opaque filters have damaged reliability and trust. Most systems today silently hide mail. Originally, e-mail either reached the recipient or became a bounce back to the sender. This ensured that important messages were never lost.
Marketers have also been ill served by the downward spiral of filter escalation. Even when they behave admirably, they find their message is too-often lost.
Your mailserver consults quorum when a message is received. A verdict is returned which is used to either reject or accept the message. If rejected, the sender gets a bounce including a link to quorum. If accepted, the recipient gets a link to quorum. These links allow the sender or recipient to ask that the mail from the sender be allowed or rejected in the future.