I'm using Lotus Domino 8.5.1 as an email program together with PureMessage but I can't make the block of senders option work for end users.
Can you please post the message_log entry of your test message.
Can you also provide the output of the following command:
$ pmx-policy inject --verbose /opt/pmx/etc/data/samples/normal/legitimate --relay=external --from BLOCKED@SENDER.com