IP Blocking: Ensure that you are using IP blocking, either at the MTA level or in the policy. The most effective way is to use IP blocking at the MTA level. This is set on the Local Services tab of the PureMessage Manager. Ensure that the IP Blocker Service is running, and then click MTA IP Blocking on the sidebar to ensure that this feature is enabled. Additionally, reverse DNS checks that detect dynamic IP addresses can be enabled. For more information, see "Enabling or Disabling MTA IP Blocking" in the Manager Reference.
A lesser, but still very significant impact on performance is to use IP blocking in the policy. This option also requires that the IP Blocker service is running. On the Policy tab of the Manager, add a main rule, setting the test as Message is from blocked IP, and ensuring that the action is Stop processing.