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This relates to a Windows SBS 2011 server and Exchange 2010.

We are seeing the following errors in Event Log:

15004:

The resource pressure increased from Normal to Medium.

The following resources are under pressure: Queue database path ("C:\Program Files\Microsoft\Exchange Server\V14\TransportRoles\data\Queue\mail.que") = 97% [Medium] [Normal=95% Medium=97% High=99%] Queue database logging path ("C:\Program Files\Microsoft\Exchange Server\V14\TransportRoles\data\Queue\") = 97% [Medium] [Normal=95% Medium=97% High=99%] Physical memory load = 88% [limit is 94% to start dehydrating messages.]

The following components are disabled due to back pressure: Inbound mail submission from the Internet Mail submission from Pickup directory Mail submission from Replay directory Content aggregation

The following resources are in normal state: Version buckets = 0 [Normal] [Normal=80 Medium=120 High=200] Private bytes = 2% [Normal] [Normal=71% Medium=73% High=75%] Batch Point = 0 [Normal] [Normal=2000 Medium=4000 High=8000] Submission Queue = 0 [Normal] [Normal=1000 Medium=2000 High=4000]

and 15006, same message as above except the first line reads:

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

From my research on this it should point to lack of available disk space on the server. However, from the 494GB C: drive there is 64GB of free space. This should be plenty of space. Can anyone give me some idea on how to resolve this issue. It has happened about 4 times in the past week.

We restart the transport service or the entire server (we aren't a big company so this isn't too big a deal for us) and this resolves the issue.

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This is known as "Back pressure" and it is a safety measure to ensure that the disk or partition that hosts the Exchange Transport database has sufficient free space to process incoming mail. Exchange monitors free space based on percentage and has the defaults described in the Event Log you shared: [Normal=95% Medium=97% High=99%]. Microsoft has an in depth article on TechNet describing back pressure in detail.

Exchange stores these files on the system drive by default. While it is possible to adjust the default thresholds, the typical (and more responsible) solution is to move the location of the Transport database to another drive.

After Exchange 2007 SP1, Microsoft included a PowerShell script to simplify and automate the process of moving the database to a different location. This script is located at C:\Program Files\Microsoft\Exchange Server\Scripts\Move-TransportDatabase.ps1 and should be run from within the Exchange Management Shell.

Of note is that there are 3 databases that can be moved by this script and I recommend moving all three. They are:

  • Transport Queue Database (Default: %ExchangeInstallDir%TransportRoles\data\Queue)
  • IP Filter Database (Default: %ExchangeInstallDir%TransportRoles\data\IpFilter)
  • Temporary Storage (Default: %ExchangeInstallDir%TransportRoles\data\Temp)

Each of these databases has an associated log that can (and should) be moved as well.

Each of the databases can be moved by specifying the appropriate parameter when running the script. The log associated with each database can be moved with its own parameter. You can identify the syntax for the script by running it with no parameters:

 [PS] C:\Program Files\microsoft\Exchange Server\scripts>.\Move-TransportDatabase.ps1

For a more reader-friendly description, Petri has a decent article on back pressure relating to Exchange 2007 as well as basic instructions for how to move the database if you do not have SP1 installed and instructions for running the script.

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