Issue: Senders report receiving bounce messages containing “452 4.3.1 Insufficient system resources”
Diagnosis: Check the free disk space on your Exchange server disk volumes.
The volume where Exchange was installed has likely fallen below the free disk space threshold. The threshold values depend on your version of exchange and range from 500MB to 4GB depending on the version of Exchange, volume size and the formula’s being used.
Immediate Solution: Either enlarge your volume, free some space, or modify the threshold value (not recommended.
Additional Detail: The Exchange Transport Queue is the service with the disk space threshold that was tripped. The Transport Queue is installed by default on the volume where you installed Exchange, which is often the C:\.
Long Term Resolution: Consider moving the transport queue (mail.que) to a volume with more available disk space where it will not be competing with the OS, Windows Updates, or the Exchange Transaction Logs.
Here are steps to move the Transport Queue on Exchange 2007:
Move the Queue Database and Queue Database Transaction Logs on Exch 2010: http://technet.microsoft.com/en-us/library/bb125177(v=exchg.141).aspx
Move the Queue Database and Queue Database Transaction Logs on Exch 2013:
and 2013: http://technet.microsoft.com/en-us/library/bb125177(v=exchg.150).aspx