Consider the threat of “theft/breach of proprietary or confidential information held in key data files on the system.” One method by which such a breach might occur is the accidental/deliberate e-mailing of information to a user outside to the organization.

A possible countermeasure to this is to require all external e-mail to be given a sensitivity tag (classification if you like) in its subject and for external e-mail to have the lowest sensitivity tag. Discuss how this measure could be implemented in a firewall and what components and architecture would be needed to do this.