ANSWER: http://technet.microsoft.com/en-us/library/bb201658.aspx Back pressure is a system resource monitoring feature of the Microsoft Exchange Transport service that exists on computers that are running Microsoft Exchange Server 2007 that have the Hub Transport server role or Edge Transport server role installed. Important system resources, such as available hard disk drive space and available memory, are monitored. If utilization of a system resource exceeds the specified limit, the Exchange server stops accepting new connections and messages. This prevents the system resources from being completely overwhelmed and enables the Exchange server to deliver the existing messages. When utilization of the system resource returns to a normal level, the Exchange server accepts new connections and messages. The following system resources are monitored as part of the back pressure feature: Free space on the hard disk drive that stores the message queue database. Free space on the hard disk drive that stores the message queue database transaction logs. The number of uncommitted message queue database transactions that exist in memory. The memory that is used by the EdgeTransport.exe process. The memory that is used by all processes. For each monitored system resource on a Hub Transport server or Edge Transport server, the following three levels of resource utilization are applied: Normal The resource is not overused. The server accepts new connections and messages. Medium The resource is slightly overused. Back pressure is applied to the server in a limited manner. Mail from senders in the authoritative domain can flow. However, the server rejects new connections and messages from other sources. High The resource is severely overused. Full back pressure is applied. All message flow stops, and the server rejects all new connections and messages.