The SMTP safe IP's do not affect relaying permissions. But of course relay permissions do have their own safe IP list and those will cause the provided IP's to send email unrestricted. As to your open backup MX, just make sure the SMTP settings POP Before SMTP and safe IP's are not allowing your connections to bypass the authentication. You should also of coures make sure you still enable SMTP Authentication (even there are no users, it restricts relaying to the provided backup domain list). If you would like me to examine your backup server configuration and logs, send the admin login details to
rob@code-crafters.com.