A delivery status notification delay means the email is pending for delivery within the email server. Email is not sent in real time, and the message is simply a notification that the delivery is taking longer than usual.
Usually, emails appear to be sent in an instant. Quite often, this is the case. However, there may be something within the system that causes a delay. When this happens, the person sending the email may receive a notification in his or her email box with the "delivery status notification delay" message.
There are a number of different reasons that might cause the lag. For instance, the recipient's email server is off line at the time delivery was attempted. In this case, the sender's email server holds the email and continue to attempt delivery for a time. Delivery is successful when the recipient's server comes back on line. Another reason is that there is a backlog in the delivery system. One small lag can cause a bottleneck of emails, especially if the server is unusually busy. An unstable server can also cause a delay. If the server is constantly experiencing technical difficulties, the sender's email server will not be able to deliver in a timely manner. Regardless of the reason, it is important to remember this is not a fatal error. Most email servers attempt delivery for five days, on average. If delivery cannot be made after that time, then the sender will receive an error message stating that the email could not be delivered.