By default an Exchange 2007 Transport Server has a queue placed in the following location, if you installed Exchange on the C-partition:

C:\Program Files\Microsoft\Exchange Server\TransportRoles\data\Queue

Because this can cause problems if your queue fills up and the system drive has no empty space, it’s wise to change the default queue to a different location. First create a directory on a different drive. In this example we change the directory to : D:\Exchange\QueueDB

You can then change the queue location of the queue database by changing the EdgeTransport.exe.config file. (location: C:\Program Files\Microsoft\Exchange Server\Bin)

Open the .config file with notepad and change the following key:

      <add key=”QueueDatabasePath” value = “D:\Exchange\QueueDB” />

Also relocate the transaction logging location for the queue database. Create a directory for the transaction logfiles. In this example we use : D:\Exchange\QueueLogs

Change the following key in the .config file :

<add key=”QueueDatabaseLoggingPath” value = “D:\Exchange\QueueLogs” />

After you changed the values, save the file. This will not have affect on Exchange 2007 yet. You must first restart the Microsoft Exchange Transport service.

For more information: Technet Article