I need to configure our sendmail server starttls to be certified by Letsencrypt. Our sendmail server is going to be relaying messages from internal servers, then connect to outside to send out bulk emails. They are servers to internal and client to external. So we call them email relay servers. Can Letsencrypt certificate be used in such situation? Is there documentation to demonstrate step-by-step setup if it can be used? The sendmail servers are CentOS 7. Also we need to have different environment, development and production. Is there any difference between the two environments in terms of certificate?