Hi,
Basically, the error is saying that the CA is unknown and the connection is dropped. If you will look into your postfix config file /etc/postfix/main.cf, you will notice something like this:
smtpd_tls_key_file = /etc/pki/tls/private/srv.servername.com.key
smtpd_tls_cert_file = /etc/pki/tls/certs/srv.servername.com.crt
smtpd_tls_CAfile = /etc/pki/tls/certs/ca-bundle.crt
smtp_tls_CAfile = /etc/pki/tls/certs/ca-bundle.crt
What you need to do is it so change the CA file path to the correct one. In my case, it was:
smtpd_tls_CAfile = /etc/pki/tls/certs/srv.servername.com.bundle
smtp_tls_CAfile = /etc/pki/tls/certs/srv.servername.com.bundle
This happened when I changed the server hostname and installed a Let's Encrypt SSL for it, from the CWP interface. For some reason, it did not change the CA file path automatically. I hope this will help someone facing the same issues.