recipients are free to ignore such requests. However, for privacy reasons, and also for backward compatibility, requests for MDNs are entirely advisory in nature - i.e. MDNs provide a notification of the "disposition" of a message - indicating, for example, whether it is read by a recipient, discarded before being read, etc. The DSN SMTP extension, message format, and associated delivery status codes are specified in RFCs 3461 through 34.
However, for various reasons, it is possible for a message to be delivered, and a DSN is returned to the sender indicating successful delivery, but the message subsequently fails to be seen by the recipient or even made available to them. Issuance of a DSN upon delivery failure is the default behavior, whereas issuance of a DSN upon successful delivery requires a specific request from the sender.
Specifically, the DSN SMTP service is used to request indications of successful delivery or delivery failure (in the DSN format) be returned. DSN is both a service that may optionally be provided by Message Transfer Agents (MTAs) using the Simple Mail Transfer Protocol (SMTP), or a message format used to return indications of message delivery to the sender of the message.
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