We recently saw that a customer received the following email message in their junk folder.
Although we are not actually sure why the customer got this message, it is an alert that the email system is trying to deliver the message and it got delayed and it will try again. It is only a warning.
Here is a short, interesting article focused on this question, which gives a little background about email systems and message delivery:
Why Am I Getting a “Delivery Status Notification (Delay)” on an Email I Sent?
If you see this problem and it persists, then at that point, it worth digging deeper to see what it is about the particular email that may cause it to be delayed (and maybe eventually fail being delivered).