When you get mail from a "MAILER-DAEMON" or a "Mail Delivery Subsystem" with a subject like "Failed Delivery" or similar, it means that an email 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 email address you're trying to send it to hasn't been closed or moved.