5.7.0 is a mismatch of your FQDN IP to the IP you are sending the eMail from. Check under Settings Network that the IP they are correct and your FQDN resolves correctly.
Your FQDN should resolve to the IP address under "Public IP" if you run an nslookup on that FQDN what does it say ? also you are using on-premise so are you using Public FQDN and local IP or split DNS ?