Spammers often use misconfigured mail servers to relay mail to their intended targets. These misconfigured servers are known as an "Open Relay". Postfix can be configured to "deny" mail from open relays using "Realtime Black Lists" (RBL). An RBL is a list of IP addresses used by spammers, any mail originating from these addresses or the "Open Relays" they use are added to this list.
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