From Microosoft Technet:

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Back pressure is a system resource monitoring feature of the Microsoft Exchange Transport service that exists on computers that are running Microsoft Exchange Server 2007 that have the Hub Transport server role or Edge Transport server role installed. Important system resources, such as available hard disk drive space and available memory, are monitored. If utilization of a system resource exceeds the specified limit, the Exchange server stops accepting new connections and messages. This prevents the system resources from being completely overwhelmed and enables the Exchange server to deliver the existing messages. When utilization of the system resource returns to a normal level, the Exchange server accepts new connections and messages.

The following system resources are monitored as part of the back pressure feature:

• Free space on the hard disk drive that stores the message queue database.
• Free space on the hard disk drive that stores the message queue database transaction logs.
• The number of uncommitted message queue database transactions that exist in memory.
• The memory that is used by the EdgeTransport.exe process.
• The memory that is used by all processes.

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I noticed this problem on my newly istalled E2k7 server. The reaseon for this is that by default MS E2k7 places all queues and databases on the installation drive… I simply moved them the correct path which had plenty of space.

I used the Management Console to move the databases and I edited EdgeTransport.exe.config to change the queue path.

If you have this problem in a lab environment you can turn this feature off like this:

– Open  EdgeTransport.exe.config, located in the C:Program FilesMicrosoftExchange ServerBin
– Change EnableResourceMonitoring to false

Note: This in NOT recommended in production.

Links

http://technet.microsoft.com/en-us/library/bb201658.aspx
http://www.exchangeinbox.com/articles/052/backpressure.htm
http://busbar.maktoobblog.com/?post=210778