tlsv1 alert unknown ca

Hi Guys, Had to replace node4 in the cluster with a new VPS. I am now seeing this in the node4 /var/log/mail.log Apr 24 15:59:28 node4 postfix/smtpd[6480]: connect from some.mail.server[1.2.3.4] Apr 24 15:59:28 node4 postfix/smtpd[6480]: warning: TLS library problem: error:14094418:SSL...