In Exchange Online and Exchange Server 2013, you can use In-Place Hold to preserve mailbox content for litigation or investigations. Many organizations also need to preserve mailbox data for users who are no longer in the organization.

http://blogs.technet.com/b/exchange/archive/2013/03/21/preserve-mailbox-data-for-ediscovery-using-inactive-mailboxes-in-exchange-online.aspx