The email address "firstname.lastname@example.org" is used by Yahoo! to notify a Yahoo! Mail user that a message failed to send. These messages are automatic and can be sent from "MAILER-DAEMON" or a "Mail Delivery Subsystem." The email usually includes a reason for the failure of delivery and the original message.
A failed message delivery can occur due to the user typing the wrong email address, the intended email address not existing, the address having been moved or because of a technical fault. When receiving a message from email@example.com, it is recommended to check the spelling of the target email address, and once corrected, resend the message.