* The value is, e.g. root@localhost <br>
     * comment: The common return path of all mail
     * @return The value of found property. (NotNull: if not found, exception but basically no way)
     */
    String getMailReturnPath();

    /**
     * The simple implementation for configuration.
     * @author FreeGen
     */
    public static class SimpleImpl extends ObjectiveConfig implements FessEnv {

        try {
            TestmailPostcard.droppedInto(postbox, postcard -> {
                postcard.setFrom(fessConfig.getMailFromAddress(), fessConfig.getMailFromName());
                postcard.addReplyTo(fessConfig.getMailReturnPath());
                stream(toAddresses).of(stream -> stream.forEach(address -> {
                    postcard.addTo(address);
                }));
                BeanUtil.copyMapToBean(dataMap, postcard);

                CrawlerPostcard.droppedInto(postbox, postcard -> {
                    postcard.setFrom(fessConfig.getMailFromAddress(), fessConfig.getMailFromName());
                    postcard.addReplyTo(fessConfig.getMailReturnPath());
                    if (toAddresses.length > 0) {
                        stream(toAddresses).of(stream -> stream.map(String::trim).forEach(address -> {
                            postcard.addTo(address);