Re: [FortiMail] The mail message logs are showing the incorrect Client IP
humm, that's a misconfiguration in the router; not directly related with fortimail.
You shouldn't see 192.168.20.1 as FEAS-CLIENT-IP header in such emails.
It is likely the router/firewall policy be NATting incoming traffic.
If your router is a FortiGate, check incoming firewall policy used to publish your fortimail. It shouldn't be NATted