UPDATE: We are making changes which should resolve issues with bounce back messages. These changes should be completed by 5:00 PM today. If you receive a bounce back message from Exchange after this please follow the instructions in these Knowledge Base articles and try again.
UPDATE: Using weboutlook.vt.edu seems to be a successful workaround for these issues at the moment. We are currently experiencing issues with the Active Directory which has caused some people to recieve email bounce back messages. This issue is being investigated.