This error is caused for one of two reasons:
- You are attempting to send email to a domain that is not recognized by this server.
- You are attempting to relay email through this server, and have not authenticated
Most likely you are seeing this message because you have not authenticated. To use this server as a relay (to send email to an outside address using your domain name), you must authenticate first.
You can do this in one of two ways:
SMTP Authentication - SMTP AUTH is a method for verifying a user's login and password before allowing Mail to be sent to other Mail Servers on the Internet. Your Email Client is required to login before sending mail. This method allows you to simply enter your Username and Password (the same ones you use for POP or IMAP) in the SMTP settings in your email program.
POP before Send - This method allows you to simply check your email before sending. Once you have checked your email, you are allowed to relay for the next 15 minutes from your current IP address without further authentication.