We are going to need more information to successfully diagnose and resolve your issue. What is your patch level on your Exchange server? What is/are the error message you get with the NDR, or have you not yet gotten an NDR? You can also open the Exchange Management Console, click on the Tools entry in the left column, and double click on the queue viewer. This will show you the status of your mail queues.In the far right hand column of inactive queue (those marked for Retry, you will see the last error message for the last message that was attempted to be sent. What is that? (This may be different than the eventual NDR message).
Go to http://www.mxtoolbox.com. Enter your domain name, and you be provided with options to get a lot of information about your e-mail situation. Wat we are interested in right now is whether you are listed on any blacklists right now. If you are, you will need to determine the reason, resolve it and then ask to be taken off the blacklist.
This is just a start. Post your answers here and Im sure we can get to the bootom of this for you.