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Exchange 2016 - Back Pressure

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In Exchange, be it Exchange 2010, Exchange 2013 or Exchange 2016, you might have come across back pressure alerts or where Exchange just won't function. This includes:

  • Disk space
  • Memory

These in turn log different events. You might see the following Event ID's logged in the application log:

  • 15004
  • 15006

If we take a look at hard disk space, you have a formula in Exchange where utilisation is calculated:

  • 100 * (hard disk size – fixed constant) / hard drive size

The value of the fixed constant is 500MB.

What happens when you reach these thresholds? Well, mail will stop flowing until the back pressure returns to normal.

You need to ensure firstly, that your disk has adequate space on it and secondly, your memory utilisation is not too high or you will also see back pressure events. Another event logged is related to version buckets. This can be due to a constraint on the Hyper-V host where the Virtual Machine is located.

Below is a sample error logged for a space issue on the Server:

Event 15006, MSExchangeTransport

Microsoft Exchange Transport is rejecting message submissions because the available disk space has dropped below the configured threshold. The following resources are under pressure:
Used disk space ("C:\Microsoft\Exchange Server\V15\TransportRoles\data\Queue")
Used disk space ("C:\Microsoft\Exchange Server\V15\TransportRoles\data")
Overall Resources

The following components are disabled due to back pressure:
Mail resubmission from the Message Resubmission component.
Mail submission from Pickup directory
Mail submission from Replay directory
Mail resubmission from the Shadow Redundancy Component
Inbound mail submission from the Internet

As you can see, a number of components above are disabled due to a disk space problem.

Once the issue has cleared, another event will be logged to say that it has dropped back down to normal and operations will resume. If you in a large organisation then the above might take a little while to clear the backlog.

Another example below is due to low available memory, take note a different Event ID (15007) is logged:

Event Type: Error
Event Source: MSExchangeTransport
Event Category: Resource Manager
Event ID: 15007
Description: The Microsoft Exchange Transport service is rejecting message submissions because the service continues to consume more memory than the configured threshold. This may require that this service be restarted to continue normal operation.

The same process applies, you either need to increase the memory on that server (if its a VM) or check what is utilising the memory. It can be Anti-Virus scans running and you need to ensure that you have the exclusions in place for Exchange. If it is a physical server, you will need to see if it is underspecified or what is using all the available memory.

Best would be to move the existing queue database and transaction logs to another location if the C:\ drive does not have enough space.

You can do so by either opening up Notepad elevated or open a Command Prompt and typing in the following:

  • Notepad %ExchangeInstallPath%Bin\EdgeTransport.exe.config

Now, find the values as shown below and change the drive letter to where it should be moved to, in this case, it was moved to E:\ as it has more space available:

  • <add key=”QueueDatabasePath” value=”E:\Queue\QueueDB” />
  • <add key=”QueueDatabaseLoggingPath” value=”E:\Queue\QueueLogs” />