In Exchange Server 2010 as with Exchange Server 2007, all e-mail must go through a Hub Transport server before delivery to the mailbox. This is mainly to allow for compliance, but it also provides a means for data recovery.
Transport dumpster is a feature built into Exchange Server 2010 HUB transport role and designed to minimize data loss during mail delivery to a DAG in a lossy failover scenario. This feature was first introduced in Exchange 2007 for CCR and LCR mailboxes.
The transport dumpster is used for replicated mailbox databases only. It doesn't protect messages sent to public folders, nor does it protect messages sent to recipients on mailbox databases that aren't replicated (not member of a DAG). The transport dumpster queue for a specific mailbox database is located on all Hub Transport servers in the Active Directory sites containing the DAG. The transport dumpster is stored inside the mail.que file.
Exchange Server Transport Dumpster Settings
There are two settings that control the life span of a message within the transport dumpster. They are:
•MaxDumpsterSizePerDatabase Defines the size available for each storage group on the Hub Transport server. The recommendation is that this be set to 1.5 times the maximum message size limit within your environment. The default value for this setting is 18 MB.
•MaxDumpsterTime Defines the length of time that a message remains within the transport dumpster if the dumpster size limit is not reached. The default is seven days.
If either the time or size limit is reached, messages are removed from the transport dumpster by order of first in, first out. we can run the following command to see the current settings:
Get-TransportConfig |fl *Dumpster*
Changes in Exchange 2010
In Exchange 2007, messages were retained in the transport dumpster until the administrator-defined time limit or size limit is reached. In Exchange 2010, 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 (mail.que) 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. After 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 smaller by maintaining only copies of messages whose transactions logs haven't yet been replicated.
The transport dumpster has also been enhanced to account for the changes to the Mailbox server role that enable a single mailbox database to move between Active Directory sites. DAGs can be extended to multiple Active Directory sites, and as a result, a single mailbox database in one Active Directory site can fail over to another Active Directory site. When this occurs, any transport dumpster redelivery requests will be sent to both Active Directory sites: the original site and the new site.