code: public SMailPostalMotorbike useStarttls() {
          if (!hasAuth) {
              throw new IllegalStateException("Not found the authenticator for STARTTLS: session=" + session);
          }
          if (MotorbikeSecurityType.SSL.equals(securityType)) {
              throw new IllegalStateException("Cannot use STARTTLS with SSL: session=" + session);
          }
          securityType = MotorbikeSecurityType.STARTTLS;
          registerSmtpAuth();
          final Properties props = session.getProperties();
          props.setProperty("mail.smtp.starttls.enable", "true");
          props.setProperty("mail.smtp.starttls.required", "true");
          return this;
      }
docs: Motorbike uses STARTTLS. <br>
  It needs authenticator, that you can specify by constructor.
  <pre>
  e.g.
  SMailPostalMotorbike motorbike = new SMailPostalMotorbike("sea", "land").<span style="color: #CC4747">useStarttls()</span>;
  motorbike.registerConnectionInfo(...);
  motorbike.registerReturnPath(...);
  </pre>
  @return this. (NotNull)