One or more messages can be placed by administrators or users in "Legal Hold." Emails classified with a Legal Hold will be placed in a second, redundant Archive, which will be kept indefinitely with no regard to the retention policies associated with those messages.
Messages can be placed in Legal Hold by searching for relevant messages, and on the search results page or the message details page, clicking on the "Legal Hold" link. Messages can be removed from Legal Hold in similar fashion. Note that any messages removed from Legal Hold that have exceeded their original retention period will be automatically removed from the Archive, and will no longer be accessible.