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.