Exchange Transport is rejecting submissions because the available disk space has dropped below the configured threshold.

Microsoft Exchange Server makes use of back pressure monitor systems to prevent Exchange Server from being overwhelmed by the volume of incoming messages.

What is Back pressure?

Back pressure is a system resource monitoring feature of the Microsoft Exchange Transport service that exists on Mailbox servers and Edge Transport servers. Back pressure detects when vital system resources, such as hard drive space and memory, are overused, and takes action to prevent the server from becoming completely overwhelmed and unavailable.

Here is an example of a back-pressure monitor kicking in, in this example the Transport service is rejecting incoming message because the disk space has dropped below the configured threshold.

To resolve this back-pressure issue, we can simply just assign additional storage to the exchange server or if that is not possible you can move the Queue DB which is located in the Exchange Install Directory “%ExchangeInstallPath%Bin” to an alternative drive.

To move the Queue DB to an alternative location, follow the below steps.

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

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:

Run the following in elevated command prompt

Your edge configuration file will open and look as follow.

Next you need to Find and modify the following keys in the <appSettings> section:

In the <appsettings> section you need to change the QueueDatabasePath and QueueDatabaseLoggingPath to the new location.

Here I will change the location to an alternative drive with enough disk space.

When you are done applying the changes, Save the Edge Configuration file and restart the services as follow.

Stop the Exchange Transport service using cmd:

Move the existing database files Mail.que and Trn.chk from the old location to the new location.

Move the existing transaction log files Trn.log, Trntmp.log, Trn nnnnn.log, Trnres00001.jrs, Trnres00002.jrs, and Temp.edb from the old location to the new location.

Once the files have been copied over to the new location, run the following to start the Exchange Transport service again.

How do you know it worked?

The new location will have transaction logs and the Queue DB file will have an updated date stamp. The old location will have no more logs and should be empty.

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