The Exchange Transport Dumpster is a feature that exists in Exchange 2007 and Exchange 2010 to prevent data loss. The transport dumpster has been revamped in Exchange 2013 and is now known as the Safety Net.
First introduced in Exchange 2007, the transport dumpster exists on the hub transport server and is only enabled for continuous cluster replication (CCR) and local continuous replication (LCR). Administrators can configure either so that after a clustered mailbox server goes down, it comes back online with limited data loss because messages stored in the dumpster are still sent to users.
Similarly, in Exchange 2010 the transport dumpster still exists on the hub transport server, but because CCR and LCR were replaced by database availability groups (DAGs), it now works with them. When a message is sent from the hub transport server to a DAG, the message is stored in the transport dumpster so in case of failure, messages can still be sent to users.