Symptoms Mail is stuck in the delivery queue with error "452 4.3.1 Insufficient System Resources"
Telneting to the 2007 Exchange server generates an error "452 4.3.1 Insufficient System Resources"
There is a feature on 2007 Exchange Hub Transport and Edge Transport servers call Back Pressure. This feature looks at system thresholds and when a threshold is exceeded it stops accepting SMTP connections.
Mail flow resumed after a reboot of the Mircosoft Exchange server. It is assumed that the threshold exceeded was a memory limit and the reboot cleared the page file.