Assuming you have got your certificates from 'certbot' successfully: Edit /etc/mail/tls/starttls.m4 Add/Edit: define(`CERT_DIR', `/etc/letsencrypt/live/examplesubdomain.example.com')dnl define(`confCACERT_PATH', `CERT_DIR')dnl define(`confCACERT',`CERT_DIR/fullchain.pem')dnl define(`confSERVER_CERT',`CERT_DIR/cert.pem') define(`confSERVER_KEY',