If you get a returned email message from a "MAILER-DAEMON" or a "Mail Delivery Subsystem" with a subject like "Failed Delivery," the message you sent was undeliverable. Failed Delivery messages are sent automatically and normally include a reason why the message was not delivered.
Returned messages contain two sections. The first section provides the reason for the return; the second section contains your original message.
The most common reason for this problem is that the email address you entered was not valid. If the returned message informs you that the email address does not exist, carefully check the spelling of the address you entered. Even a single misplaced letter will cause a failed delivery. If the problem remains, the account might have been closed or moved.
If the email address is valid and the recipient's account is still active, send an email message to your Web Mail System Administrator and include the reason the delivery failed.