Exchange 2007 introduces a concept called back pressure. This is a monitoring feature in the Exchange transport service that watches system resources like available disk space and memory. If a resources exceeds a specified limit, Exchange stops accepting new connections and messages so that it can deliver the existing messages without being completely overloaded. Exchange starts accepting new connections and messages once resources have returned to normal levels. A large number of event log messages about the back pressure situation are logged. We had this problem at a customer site recently and had to increase the RAM of the Virtual Machine to fix it. [more]
View the TechNet article on Back Pressure for more details.