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. Specifically, the DSN SMTP service is used to request indications of successful delivery or delivery failure (in the DSN format) be returned. 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.