Thanks for your reply.
I guess I understand it a little bit better now. With my configuration Discourse already created the LE certificate while installing. The next step to get this running would be using that certificate on my mailserver which seems to be kinda tricky. At least I didn’t found anyone mentioning a process like this. I searched for pem files in the Docker VM and found them in /etc/ssl/certs and in ruby-folders under /usr/local/lib and /var/www/discourse/vendor. But I don’t really know if I could just copy them to the virtual machine that’s running the mailserver.
So the conclusion is, that it’s not possible to make this setup run without having more knowledge about TLS.