|0||Send notificaton at the server discretion. Usually, this is equivalent to eitheror bitwise combination of and .|
|1||Notify if message delivery fails.|
|2||Notify if message delivery is delayed.|
|4||Notify if the message delivery succeeds.|
|7||Notify always. This value is a bitwise combination of, , and .|
|8||Do not send notification under any circumstances.|
If the SMTP server the message it submitted to supports ESMTP DSN (Delivery Status Notification) extension, it's possible to tell the server to send notifications on message delivery (successful, failed, or both) to the message sender.
If the SMTP server does not support ESMTP DSN, this value is ignored.