A domain with Backup MX should have the MX records shown below, your primary MX record that points at your mail server must have the lowest number (highest priority). mail1.no-ip.com must also have a lower number than mail2.no-ip.com.
Example:
Host | Priority |
mail.domain.com | 5 |
mail1.no-ip.com | 10 |
mail2.no-ip.com | 15 |