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Back-pressure is used to monitor resources like hard disk space , availability of memory and version buckets to give an advance notification to the administrator before the email server is totally down.This feature was introduced from Exchange 2007. The concept of back-pressure in Exchange 2013 is the same as it was in Exchange 2010.
Basically high level of hard drive space utilization is calculated by using the following formula in Exchange 2013:
100 * (hard disk size – fixed constant) / hard drive size
The value of fixed constant is 500 megabytes (MB)

A list of changes that are made to the message queue database is kept in memory until those changes can be committed to a transaction log. Then the list is committed to the message queue database itself. These outstanding message queue database transactions that are kept in memory are known as version buckets.

If normal level isn’t reached for the entire version bucket history depth, then edgetransport.exe config file is coded to take the following actions:

1) Reject incoming messages from other Exchange servers ( could be internal exchange servers as well as external exchange servers)- initially

2) Reject message submissions from mailbox databases by the Mailbox Transport Submission service on Mailbox servers – End users sent email received from their respective databases to transport submission service will be rejected. Which means that these messages will not reach till the categorizer level and all the messages will be rejected at the precategorizer level itself.
3) Reject incoming messages from non-Exchange servers – Could be notes,zimbra etc.,
4) Reject message submissions from Pickup and Replay directories – Messages from applications dropped in the pickup directory

Similarly following event logs will be logged in the affected server:

Event log entry for an increase in any resource utilization level
Event Type: Error
Event Source: MSExchangeTransport
Event Category: Resource Manager
Event ID: 15004
Description: Resource pressure increased from Previous Utilization Level to Current Utilization Level.

Event log entry for a decrease in any resource utilization level

Event Type: Information

Event Source: MSExchangeTransport

Event Category: Resource Manager
Event ID: 15005
Description: Resource pressure decreased from Previous Utilization Level to Current Utilization Level.
Event log entry for critically low available disk space
Event Type: Error
Event Source: MSExchangeTransport
Event Category: Resource Manager
Event ID: 15006
Description: The Microsoft Exchange Transport service is rejecting messages because available disk space is below the configured threshold. Administrative action may be required to free disk space for the service to continue operations.
Event log entry for critically low available memory
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.

So the above event logs will help you to identifying the back-pressure on the affected server

Solution:

Use the Command Prompt to move the existing queue database and transaction logs to a new location.
In a Command prompt window, open the EdgeTransport.exe.config file in Notepad by running the following command:

Notepad %ExchangeInstallPath%Bin\EdgeTransport.exe.config

Change the drive letter in which you wish to have the new location by changing in the below add key values

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

Thanks 

Sathish Veerapandian

MVP – Exchange Server