This post describes Back Pressure symptoms. If you can verify Back Pressure as the cause of your problem after reading this you can find more here on how to prevent Back Pressure Insufficient System Resources.
Situation : Mails sent to recipients on your Exchange Server will be delivered with a huge delay or or will not be delivered at all. In the mail-logs of the last hop Message Transfer Agent ( MTA ) you see :
452 4.3.1 Insufficient system resources.
You can verify Back Pressure symptoms as follows …
In the event logs of your Exchange Server you will find one of the following events :
Event ID: 15004
Event log entry for an increase in any resource utilization level
Event ID: 15006
Event log entry for critically low available disk space
Event ID: 15007
Event log entry for critically low available memory
We were sure our server’s C drive [ 100GB ] had still 18 GB free space and this did not change much for a long couple of month ( since we have outsourced the .mail.que file to another drive. ) At this point Back Pressure could be taken into consideration as the culprit. Since Back Pressure requires the Exchange installation drive ( in our case C: ) to have at least 20% free space. That means in this case at least 20GB free space.
Another possibility causing Back Pressure can be the memory used. But see the criteria and thresholds yourself in the output from the command below.
First you need to add the ExchangeSnapIn for Powershell or ISE.
To verify Back Pressure status run :
$bp=Get-ExchangeDiagnosticInfo -Process EdgeTransport -Component ResourceThrottling; $bp.Diagnostics.Components.ResourceThrottling.ResourceTracker.ResourceMeter
You will get an Output ( always from the server where you trigger the cmd ) like :
Resource : SystemMemory
CurrentResourceUse : Low
PreviousResourceUse : Low
PressureTransitions : [PressureTransitions: MediumToHigh=94 HighToMedium=89 LowToMedium=88 MediumToLow=84]
Pressure : 38
Resource : UsedDiskSpace[C:\Program Files\Microsoft\Exchange Server\V15\TransportRoles\data]
CurrentResourceUse : Low
PreviousResourceUse : Low
PressureTransitions : [PressureTransitions: MediumToHigh=99 HighToMedium=90 LowToMedium=89 MediumToLow=80]
Pressure : 55
This is just an example. The output contains more parameters you can verify. But here you may get an idea what causes the delays.
Regarding disc space Back Pressure starts straight away when the MediumToLow threshold has been reached !
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