Exchange 2010 Back Pressure

Back pressure is a system resource monitoring feature of the Microsoft Exchange Transport service that exists on Microsoft Exchange Server 2010 Hub Transport and Edge Transport servers. Exchange transport can detect when vital resources, such as available hard disk space and memory, are under pressure, and take action in an attempt to prevent service unavailability.

Back pressure prevents the system resources from being completely overwhelmed, and Exchange tries to deliver the existing messages. When utilization of the system resource returns to a normal level, the Exchange server gradually resumes normal operation.

Reference here and here;
Nice Powershell tool here;