This error message means that the application you are using for sending mails does not comply with corresponding RFC standards. This is a common problem when sending emails directly from your web site.
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 use another application for sending mails or use an account on a properly configured email server. All our shared email servers are properly configured and RFC compliant.