When you get mail from a "MAILER-DAEMON" or a "Mail Delivery Subsystem" with a subject similar to "Failed Delivery," this means that a message you sent was undeliverable and has been "bounced" back to you. These messages are produced automatically and usually include a reason for the delivery failure.
Bounced messages normally consist of two parts. The first contains the reason for the bounce; the second contains your original message.
The most common reason for a failed delivery is that the email address you entered isn't valid. If the Mailer Daemon says that the account doesn't exist, double-check the spelling of the address you entered. A single misplaced letter can be enough to cause a failed delivery. If the message continues to be bounced back to you, you might want to verify that the account hasn't been closed or moved.
If there's a different reason given and you'd like us to investigate it, just let us know. If you can copy and paste the reason given for the failed delivery into the message you send us, it will be very helpful as we attempt to determine why your message bounced.