mail-proxy starttls and ssl on (no replies)

The manual

Setting up SSL/TLS for a Mail Proxy

https://www.nginx.com/resources/admin-guide/mail-proxy/

says

Enable SSL/TLS for mail proxy with the ssl directive. If the directive is specified in the mail context, SSL/TLS will be enabled for all mail proxy servers. You can also enable STLS and STARTTLS with the starttls directive:
mail {
...
ssl on;
starttls on;
...
}


However if I add both,


nginx: [warn] "ssl" directive conflicts with "starttls" in /root/nginx.conf:79
nginx: configuration file /root/nginx.conf test failed

How comes?