Use the Retention page to configure the default retention period for messages in the repository:
If the period is not indefinite, then once a message was created more than this period ago, it is no longer found in a search by users, message links report that the message has expired, and the next time an archive policy runs on the mailbox the message link is deleted. A standard administrative search does not find the item unless the search status is set to search expired items. An administrator can update the retention period for a group of mails. For more information, see Archive Updates.