a message that did not reach the user for a number of possible reasons
A returned, can't deliver e-mail message.
An electronic mail message returned with a notice indicating the transmission failed, either because the message was misaddressed or a connection failed. Possible reasons for this are temporary server outages and peak or busy times. This can account for you receiving an email message that says your email was undelivered when in reality it is delivered on the next attempt by the server.
An email that is returned to the sender because the email was not deliverable (usually because the email address does not exist).
A returned, undeliverable email message.
an e-mail message that has been returned to the sender because of an error of some kind.