ISPconfig does this exact test and that's why you see that the checkboxes get disabled automatically when it is not possible to reach the domain on that server. Off course, this takes about a minute as this test has to be done on the server itself and not on the interface. And that you can not create an LE cert for a domain not pointing to your server in DNS is not related to ISPConfig at all, It's LE that is not able to verify the domain and rejects the SSL cert and ISPconfig shos you that.
Ah ok. Yes I do know that it is LE. In the past I have done it manually. Thanks to both of you for your assistance and comments.
