Prerequisites

Admin (root) access to Kerio Connect server

Root Cause

Kerio Connect was picking the self-signed certificate instead of the valid SSL.

Stop Kerio Connect.
Navigate to the Kerio Connect installation folder. Defaults are:
- Windows: C:\Program Files\Kerio\MailServer
- macOS: /usr/local/kerio/mailserver
- Linux: /opt/kerio/mailserver
Rename sslcert folder to sslcert.OLD.
Create a new folder and name it sslcert.
Copy the valid certificate and key from sslcert.OLD, as shown in the image below, and move it to the sslcert folder.
Start Kerio Connect.

SSL certificate is now showing as valid.