certification bug

Hello, I am with gcore to manage my domain name and I have the problem of incorrectly configured SSL/TLS.

For which protocol? Because I do not get a certificate error in my browser:

My guess would be that your OS is not trusting the LetsEncrypt root certificate. Are you sure you’re running a supported OS with latest updates?

hello my mail domain is
serveur.root-network.fr
client.root-network.fr
and
admin.root-network.fr
i use outlook (clasic)

These are not mail domains. These are hostnames.
And you did not answer our questions.