We removed this message from the console. It is possible to install a free certificate as per
http://help.mailarchiva.com/enterprise-certificates$freecert. It does require that you switch MailArchiva to run on port 80, then port forward 80 from an external IP on your firewall to MailArchiva. Then, create an outside DNS entry that refers to this IP address. Let's encrypt will retrieve and install a certificate automatically. Then you can switch Tomcat to use port 443. If this sounds all too complex, you could front Mailarchiva with Nginx or Apache (see:
http://help.mailarchiva.com/nginx). Failing that, I suggest you consider using the MailArchiva Cloud service as this has everything setup.