static void smtp_client(CLI *c) {
    char *line;

    do { /* copy multiline greeting */
        line=fdgetline(c, c->remote_fd.fd);
        fdputline(c, c->local_wfd.fd, line);
    } while(isprefix(line, "220-"));

    fdputline(c, c->remote_fd.fd, "EHLO localhost");
    do { /* skip multiline reply */
        line=fdgetline(c, c->remote_fd.fd);
    } while(isprefix(line, "250-"));
    if(!isprefix(line, "250 ")) { /* error */
        s_log(LOG_ERR, "Remote server is not RFC 1425 compliant");
        longjmp(c->err, 1);
    }

    fdputline(c, c->remote_fd.fd, "STARTTLS");
    do { /* skip multiline reply */
        line=fdgetline(c, c->remote_fd.fd);
    } while(isprefix(line, "220-"));
    if(!isprefix(line, "220 ")) { /* error */
        s_log(LOG_ERR, "Remote server is not RFC 2487 compliant");
        longjmp(c->err, 1);
    }
}

static void smtp_server(CLI *c) {
    char *line;

    s_poll_init(&c->fds);
    s_poll_add(&c->fds, c->local_rfd.fd, 1, 0);
    switch(s_poll_wait(&c->fds, 0, 200)) { /* wait up to 200ms */
    case 0: /* fd not ready to read */
        s_log(LOG_DEBUG, "RFC 2487 detected");
        break;
    case 1: /* fd ready to read */

        s_log(LOG_DEBUG, "RFC 2487 not detected");

        return; /* return if RFC 2487 is not used */
    default: /* -1 */

        sockerror("RFC2487 (s_poll_wait)");
        longjmp(c->err, 1);
    }

    line=fdgetline(c, c->remote_fd.fd);
    if(!isprefix(line, "220")) {
        s_log(LOG_ERR, "Unknown server welcome");
        longjmp(c->err, 1);
    }
    fdprintf(c, c->local_wfd.fd, "%s + stunnel", line);
    line=fdgetline(c, c->local_rfd.fd);
    if(!isprefix(line, "EHLO ")) {
        s_log(LOG_ERR, "Unknown client EHLO");

        longjmp(c->err, 1);

    }
    fdprintf(c, c->local_wfd.fd, "250-%s Welcome", line);
    fdputline(c, c->local_wfd.fd, "250 STARTTLS");
    line=fdgetline(c, c->local_rfd.fd);
    if(!isprefix(line, "STARTTLS")) {
        s_log(LOG_ERR, "STARTTLS expected");

        longjmp(c->err, 1);

    }
    fdputline(c, c->local_wfd.fd, "220 Go ahead");
}

After the initial handshake, it looks for the STARTTLS line. From there, that's where the magic happens. Try setting protocol equal to smtp.