This message is sent back to you, the sender, from the SMTP server. The server is telling you that the "host" you tried to send e-mail to does not exist. The "host" is just the domain name proceeding the "@" in the e-mail address. In the case of jason@fastwebhost.com, the host would be fastwebhost.com.

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.