This is a message delay notification, and it is sent by our servers to let you know when any of your messages are delayed for any reason. The mail body will contain the precise reason for the delay in delivering the message in question.
Our servers are set to send you message delay notifications quickly, so that you know about any delivery delays as soon as possible. Our servers will send you a message delay notification if any message you sent has been delayed for 60 minutes. Delivery attempts will continue automatically, and you're kept informed of any further delays with further delay notifications. If you don't receive a delay notification for any message within 1 hour, then you can be sure that the message was delivered.
Some mail servers use a method called "grey-listing" to cut spam, which involves temporarily rejecting an incoming message the first time a message is received from a previously unseen sending server, and accepting it on the next delivery attempt. This can cause messages to sometimes be delayed for 60 minutes or more, which will result in your receiving a message delay notification. But this is usually nothing to worry about, unless you get repeated delay notifications for the same message over a period of time.