I have seen this on ours and disabling the orfilter and then restarting IIS seems to help. 

This seems to happen after a large number of emails come in or a very large email comes in.  We finally went through and adjusted some of the matches (regular expressions) and this seemed to help reduce the number of times this occurs.  It is possible that depending on your regular expressions, some are recursing way too much, tying up the system trying to complete it.