When you see an email from some Mail Delivery Subsystem or a MAILER DAEMON, along with a subject line that says Failed Delivery or something along those lines, it implies that your sent email has come back to you as it was undelivered at its intended address.
The message you received is an automatically generated response which will also tell you the reason for the failure in its sending. The email you sent becomes a bounced message and will be divided in two parts. The first part will tell you the reason for the bounce, while the second part will show you the original message as you had sent it.
The most obvious, and most common reason for a delivery failure is the invalidity of the email address to which you had sent the mail. This means that the first part of the bounced email will tell you that the account you tried to send your mail to does not exist. So, double check the address, while paying careful attention to spelling and placement of any special characters. A single letter, if not placed in its correct spot, can cause a failure in the mail delivery.
If this keeps happening and the message continues to bounce, it is best for you to recheck the address and make sure that it hasn’t been deleted, moved or closed.