I’ve recently had some issues with new installations of Exchange servers that were trying to send out NDR responses to spammers, which quickly filled the Exchange Queues and delayed emails.
This article explains what and why this happens, and also helps you fix the problem.
http://www.experts-exchange.com/Software/Server_Software/Email_Servers/Exchange/A_4257-Exchange-2007-2010-Backscatter-and-how-to-resolve-it.html