This error message means that the application you are using for sending mails is not properly configured as an email server. This is a common problem when sending emails directly from your web site using the in-built SMTP service.
All scripting languages support sending emails via an account on a remote email server. You should ALWAYS use this method to send emails from your web site and NEVER send emails directly via the web server.
Although this happens rarely, the only solution is to make sure you use an account on a properly configured email server for sending outgoing emails.
This is not an email server error but an error in the way you are sending your emails