Why is "Secure webmail on this domain" the only option when using Let's Encrypt?
There are two possible reasons:
1. The Domains > example.com > Hosting Settings > SSL/TLS Support option is disabled. This option should be enabled in order to secure the domain.
2. The Domains > example.com > Hosting Settings > Hosting Type option is set to No Hosting. Since the domain is not actually hosted on the server, it is only possible to secure the webmail component. Other hosting types like Website Hosting and Forwarding allow a full range of options for securing the domain.