Back pressure can cause serious problems in an Exchange Server environment due to the interruptions it causes to message delivery. Be sure to check your Transport servers for signs of back pressure, and take steps to resolve the underlying issues.

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For most Exchange administrators the first time they encounter the concept of “back pressure” is when they see this error:

452 4.3.1 Insufficient system resources

They might see it for the first time in a non-delivery report, an SMTP error log from an application, a telnet session, or the queue viewer on another Exchange server.

In this article:

An overview of Transport service resource monitoring
Customizing back pressure thresholds
Detecting back pressure
Monitoring Transport queues
Monitoring event logs
Monitoring protocol logs

Microsoft Exchange Transport Service Resource Monitoring

Back pressure is the name for a condition that an Edge Transport or Hub Transport server is in when it is in an overloaded state and is actively refusing some or all further connection attempts from other systems.

The overloaded state is based on a series of resource utilization metrics:

Free disk space on the drive(s) that store the message queue database and logs
Uncommitted queue database transactions in memory
Memory utilization by the EdgeTransport.exe process (the Microsoft Exchange Transport service)
Overall memory utilization for the server

Each of those metrics is measured individually, and as such each is individually capable of causing the server to go into a back pressure state. There are two different levels of back pressure. as well as the condition where no over-utilization is occurring, so in total there are three resource utilization conditions that your Edge or Hub Transport servers can be in:

Normal – all is well and the server is performing its role as intended (assuming you haven’t modified the back pressure settings to mask a genuine problem – more on that later)
Medium – a resource is moderately over-utilized and the server begins limiting some connection types. Typically internal email flow remains functional while email from external or non-Exchange sources will be rejected.
High – a resource is severely over-utilized. The server ceases to accept any new connections.