Doug - the traffic looks to be DNS lookup traffic. Are there a large number of incoming/outgoing deliveries when this happens? Both incoming/outgoing deliveries and the associated spam checks will generate DNS lookups and server load while those are taking place, and blocking DNS lookups would cause the delivery process to break down.
Kyle Kerst
System/Network Administrator
SmarterTools Inc.
(877) 357-6278
www.smartertools.com