If you tried to send a letter via postal mail to a non-existent address, the mail would eventually come back to you with "address unknown" stamped on it by the post office. Similarly, if the host does not exist, then e-mail cannot be delivered, and it is returned to the original sender with a similar message.
You will need to verify the spelling of the domain name. Make sure this domain name exists (is registered and in use) otherwise your e-mail will continue to bounce.
