Mike, the user will receive this message if they have been flagged as a potential internal spammer according to your Internal Spammer Abuse Detection rule. If they are a genuine user and you know their IP address, you may want to whitelist them. Currently, there is no list containing blocked internal spammers. In the rule, you can set up an email address to notify (like the domain admin), block the user outright, or quarantine the sender's messages.