I don't think many Exchange 2010 Production environments will run into this issue but this may be helpful to someone running in Dev/Test environment where resources may be somewhat limited.

In Exchange 2007 and 2010 the Transport service monitors resources like disk space and memory.  If resources hit a specific threshold the service will stop receiving email.  You will be able to send email but not receive.  For example the default value for free disk space for queue database is 500MB.

You can find more information here: http://technet.microsoft.com/en-us/library/bb201658.aspx

The one thing I don't like is that when this happens event viewer is not always helpful and if you test connectivity using Microsoft Remote Connectivity Analyzer the error makes it seem more like a disk space issue.  See image below.

In my case the memory use was at 92% on my Edge Transport server.  Since I run VM's it was a quick fix to add an additional 1GB memory.  Server booted up and email started to flow normally.  It took me a little while to figure out it was memory as the first thing I checked for was low disk space but I had over 20GB free on the data disk.  Email had also been flowing correctly for several months.  But once I thought to check memory usage it was clear the system needed more RAM.