In Exchange 2007 a new role was deployed called the Hub Transport Server, to make a long story short the Hub Transport Server was responsible for routing all emails inside and outside of an organization.  For redundancy purposes the Hub Transport Server has a feature called the Transport Dumpster.  The Transport Dumpster was responsible for keeping a record of emails which passed through the Hub Transport until the size or time limit was exceeded which was specified by the administrator, in which case they would be removed from the queue.   The Transport Dumpster was only used for redundant solutions such as a CCR deployment.  In the event of a failure the CCR passive node would query the Transport Dumpster to validate that all emails in the database were valid and up to date.

The function of the Transport Dumpster has changed in Exchange 2010.  When using DAGs the transport dumpster now receives feedback from the replication pipeline to determine which messages have been delivered and replicated. As a message goes through Hub Transport Servers on its way to a replicated mailbox database in a DAG, a copy is kept in the transport queue until the replication pipeline has notified the Hub Transport server that the transaction logs representing the message have been successfully replicated to and inspected by all copies of the mailbox database. Once the logs have been replicated to and inspected by all database copies, they are truncated from the transport dumpster. This keeps the transport dumpster queue leaner by maintaining only copies of messages whose transactions logs have not yet been replicated.

I was able to find out this information and more at: http://technet.microsoft.com/en-us/library/dd638137(EXCHG.140).aspx