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.
In Exchange Server 2007, when a Hub Transport or Edge Transport server is under resource pressure, it rejects incoming connections. In Exchange 2010, incoming connections are accepted, but incoming messages over those connections are either accepted at a slower rate or are rejected. When an SMTP host attempts to make a connection to a Hub Transport or Edge Transport server that’s in back pressure, the connection will succeed but when the host issues the MAIL FROM command to submit a message, depending on the resource that’s under pressure, Exchange either delays the acknowledgement to the MAIL FROM command or rejects it.
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