Mailer-daemon is an email server designed to deliver email messages to designated recipients. When a mailer-daemon message is delivered to an inbox, it usually indicates that an email message sent from the inbox could not be delivered.
One may receive a mailer-daemon delivery failure notice for several different reasons. The recipient email address may no longer be valid, or there may be a routing problem with the sent message. A delivery failure message is also sent when a domain is on a blacklist, meaning the server will refuse all email messages coming from it. If the recipients inbox is full, a mailer-daemon message may be sent.