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Another episode about an issue we had with our Exchange 2007 setup. From one day to the other, Exchange stopped to recieve mails from our mail gateway. It refused every connection with the following error code:

452 4.3.1 Insufficient system resources

But the rest of the system was still working as usual (Outlook clients were connected, internal mails (Exchange - Exchange) worked pretty well). So it was only the transport service which is responsible for recieving and sending mails via SMTP. Restarting the transport service and flushing the queue on the mail gateway worked for approx. 20 mails - then the same error occured again and any further mail was put back to the queue on the gateway.

Reason for this issue

Exchange 2007 keeps it's queue database and transaction log file on drive C:\ by default. The transport service continously monitors the system's vital details like memory usage, free disk space, temperature and wheather condition (just kidding). If now the free disk space on drive C:\ falls below 4 GB, the transport service takes this as a trigger to stop working and just rejects all incoming mails. Outgoing mails still work!

In our case, the disk had around 4 GB - sometimes a bit more, some times a bit less. That's also the reason why we had that issue for some hours, then it vanished, then it came back, ...

Now we could fix it the following way:

Solution for this issue

It's as simple as it sounds: Just move that queue database do another drive where more space is available. In our case it's drive D:\ which also contains the whole mailbox store.

Steps to take in a short overview:

• Stop the transport service
• Create the folder where the queue should be stored
• Edit the config file to reflect the new location of the queue database
  • Open this file: C:\Program Files\Microsoft\Exchange Server\Bin\EdgeTransport.exe.config
  • Search for "QueueDatabasePath" and set this option to the new path where the queue should be stored
  • Do the same for "QueueDatabaseLoggingPath" if you also want to move the transaction los files
• Start the transport service
• Do the usual testing to verify those changes

For the details, refer to the documentation at Microsoft Technet on this topic. Or read the article where I found some insight to fight this issue.