-- status=deferred
-- delivery queue
-- bounced

All of a sudden inbound emails started queueing up in the delivery queue and the Message Tracking is showing the following error under the Sending Trace option:

host mx.domain.com[XXX.XX.XX.X] said: 452 4.3.1 Insufficient system resources (in reply to MAIL FROM command)

What might be casing this?

This happens when the physical server resources level, usually HDD space, drops below a specified limit.

In Exchange Server 2007 and 2010, the Transport service monitors system resources such as disk space and memory and stops message submission if it is running low on these resources.

However, it continues to deliver existing messages in the queue. When resource utilization returns to normal.