If you find that email stops flowing on your Exchange 2007 server, and you find an alert within the event log stating: 452 4.3.1 Insufficient system resources, it’s because the disk drive your queue database is stored on is below the threshold of 500MB or 4GB if you’re using Exchange 2007 without SP1.

By default the queue database needs to be stored on a server with 4GB of free space. But this was changed to 500MB as part of Exchange 2007 Service Pack 1.

Email will stop flowing if disk space is below the threshold of 500MB. This is known as Back Pressure. More details on understanding back pressure can be  found at Understanding Back Pressure

To resolve this issue, you can do one of the following: (Always make sure you have a backup of your system before making any changes)

1) Clear disk space so you have more then 500MB of free space remaining
2) Disable back pressure so you no longer have a threshold (Not recommended by Microsoft, but it will allow you time to sort your issue whilst mail continues to flow)
3) Move the queue database to a bigger drive

Disable Back Pressure

To disable back pressure follow the instuctions below (This is not recommend by Microsoft but will bring your server backup whilst you investigate)

1) Access the drive where your queue database is stored on . It’s usually the C drive.
2) Access C:\Program Files\Microsoft\Exchange Server\Bin
3) Locate the file EdgeTransport.exe.config and open with Notepad (unselect ‘Always use the selected program to open this kind of file’) before opening
4) Locate <add key=”EnableResourceMonitoring” value=”true” /> and change to false
5) Save
6) Restart the Microsoft Exchange Transport Service

For more details see: Understanding Back Pressure

Move the queue database

To move the database follow the instructions below:

1) Access the drive where your queue database is stored on . It’s usually the C Drive.
2) It’s usually C:\Program Files\Microsoft\Exchange Server\Bin
3) Locate the file EdgeTransport.exe.config and open with Notepad (unselect ‘Always use the selected program to open this kind of file’) before opening
4) Add the code below if it does not already exist. Add below section:

<add key=”QueueDatabasePath” value=”C:\Queue\QueueDB” /> (Amend the path’s before saving)

5) Restart the Microsoft Exchange Transport Service

For more details see: Change the Location of the Queue Database