Just when I thought everything was going so well, a manager did not receive an important email. Also, the sender (another important person) didn't receive the NDR. In troubleshooting, I have found the following information: The sender sent a message with a large attachment. The recipient addresses appear correct, so we are currently assuming the attachment was over the size limit. The Message Tracking Center shows the message details, with an indication that an NDR was generated. The SMTP log shows entries indicating that the NDR was sent. Here is the first weird thing: the NDR message does not appear to contain a from address. That is, the SMTP log shows "MAIL - FROM <>". How do I fix that? I am suspecting that as the reason the NDR was not delivered -- it was probably dropped by the recipient server as a null-sender message. The second problem is that I don't have enough forensic evidence to troubleshoot the original problem. The sender claims the attachment was only 7.4 MB. Our inbound limit in the Message Delivery, received message size max is 20 MB, and the SMTP limit message size is not checked at all! (I'm not sure which parameter applies.) How do I determine the cause of the NDR? Oh - one more thing - I stupidly had no location set for the delivery of NDRs. So I can't see what happened. I have changed that so that the mail administrator account receives a copy of the NDRs. I guess that includes outbound ones.