Microsoft Exchange 2013 Safety Net is a new feature in Exchange Server 2013 that prevents data loss by maintaining a queue of successfully delivered messages.
The Exchange 2013 Safety Net is essentially a delivery queue that exists on each Exchange 2013 mailbox server. Each time a message is delivered to a database, a copy of that message is held within the Safety Net. The message copy remains in the Safety Net until its expiration date, which is determined by the administrator.
The Safety Net is essentially an improvement of Exchange 2007 and Exchange 2010's Transport Dumpster feature, though it does not require database availability groups. Additionally, unlike the Transport Dumpster, the Exchange 2013 Safety Net is not a single point of failure.