With the on-going war against spam, we were finding that 85% of all spam we were receiving was directed to our default web mailbox (as oppose to individual user email accounts) such as email@example.com. And of those, the majority used bcc to include our address which doesn't display the recipient in the To or CC fields. So, we added an "after new mail has arrived" agent which deletes email that do not contain "mycompany" in the To, CC, BCC or From fields. Users never see the emails. The downside of this fix is that the server still processes the email then deletes them as oppose to reject them as soon as they arrive. You can also have this agent move the email to a "spam" folder for administrators to view.
SEARCH CRITERIA: field SendTo does not contain mycompanyANDfield cc does not contain mycompanyANDfield BlindCopyTo does not contain mycompanyANDfield From does not contain mycompany
ACTION: Delete document