The following table contains the NDR codes (also called enhanced status codes) for the most common bounce messages and NDRs that you might encounter in Exchange Online. Find my NDR code and get help delivering my email NDRs include an error code that indicates why your email wasn't delivered, solutions to help you get your email delivered, a link to more help on the web, and technical details for administrators. Non-delivery can be caused by something as simple as a typo in an email address. The most common type is called a non-delivery report (NDR) and they tell you that a message wasn't delivered. The email you receive is a delivery status notification, also known as a DSN or bounce message.
When there's a problem delivering an email message that you sent, Microsoft 365 or Office 365 sends an email to let you know.
