A new feature of Exchange Server 2007 that helps prevent the inundation of system resources of an Exchange Server 2007 transport server is back pressure.
Back pressure is a system resource monitoring feature of the Exchange Transport service that exists on computers that are running Exchange Server 2007 that have the Hub Transport server role or Edge Transport server role installed.
When a monitored system resource, such has hard disk drive utilization or memory utilization, exceeds the specified threshold, the Exchange transport server stops accepting new connections and messages, and concentrates on delivering existing messages. This prevents the system resources from being completely overwhelmed and enables the Exchange server to deliver the existing messages. When the utilization of the monitored system resources returns to normal levels, the Exchange transport server accepts new connections and messages.
For each monitored system resource on a Hub Transport server or Edge Transport server, the three levels (Normal, Medium and High) of resource utilization are applied.
For example, by default, the message queue database is stored at <drive letter>:Program FilesMicrosoftExchangeServerTransportRolesdataQueue. By default, the high level of hard disk drive space utilization is calculated by using the following formula: 100*(hard disk drive size - 4 GB) / hard disk drive size. As the available free hard disk drive space decreases, the hard disk drive utilization increases. So, we require at least 4GB free space on the hard disk drive containing message queue database. Otherwise, the hard disk drive space utilization will reach the high level and Exchange stops accepting any new connections and messages.
People who are not aware of this new back pressure feature can be surprised when they find that the mail flow stops their servers.
For more information about and better understanding the back pressure feature, please view the Microsoft TechNet article at http://technet.microsoft.com/en-us/library/bb201658.aspx