This is not a new term, but a new use for an existing one.
Fail Mail: When a user's email gets stuck in their Outbox and refuses to be sent, for any reason.
John found out why Julia had not received his invitation. There it was, stuck in his Outbox. His beautifully written, sweet, sincere invitation to the girl of his dreams had turned into fail mail.
Free Daily Email
Emails are sent from firstname.lastname@example.org. We'll never spam you.