Delivery Status Notification (DSN) has been around since RFC 821 (1982). Once the DATA portion of the SMTP protocol is completed and the server accepts an email for delivery, the DSN is responsible for it. If for some reason the email does not reach the recipient, the DSN should return it to the original sender with a notification of the failure.
IIS SMTP Configure the content of the delivery status notification
This old convention meant that you either got an error message or nothing at all. The email might have arrived or it might not. The error messages were in many cases just as useful as no error messages.
RFC 1891 proposes several extensions to the SMTP protocol that should result in a more reliable and usable DSN system. It is a set of extensions to the MAIL and RCPT commands.
First, check if the server supports DSN, say EHLO and listen carefully. If DSN is listed somewhere in the feature list, the server can handle requests. If not, try another server or switch back to email without DSN. For example:
