Outlook 2007 is getting an NDR message when sending email externally. It begins intermittently, some addresses have successful delivery, others do not. Within three day, all external outgoing mail receives NDR. The user doesn't experience this problem after disabling cached mode or when using Webmail (Exchange 2007). I deleted the OST, but that did not solve the problem so I deleted the Outlook profile and created a new one. This worked for a short time. Any suggestions?