Why does enforcement sometimes fail on messages sent to a distribution list?

When the Agari sensor is told to enforce on a message, it runs an API call for that message ID in the recipient's mailbox. If the recipient is a distribution list without an actual mailbox the enforcement action will fail as the message does not technically exist. Agari will expand the distribution list in most cases, and the messages going to the end-users actual mailbox will be enforced on.