I am running a Debian 8 Jessie email server with postfix and dovecot. I wanted to change from self signed certificates to certificates from Let’s Encrypt. It was easy to do and I wish had not waited so long to change. This blog post, https://www.shivering-isles.com/lets-encrypt-free-trusted-certificates-for-postfix-and-dovecot/ explained it perfectly for me, so there is no need for me to document the commands.