* comment: The prefix of subject to show test environment or not
     * @return The value of found property. (NotNull: if not found, exception but basically no way)
     */
    String getMailSubjectTestPrefix();

    /**
     * Get the value for the key 'mail.return.path'. <br>
     * The value is, e.g. root@localhost <br>
     * comment: The common return path of all mail

        // Test getMailSmtpServerMainHostAndPort
        assertEquals("localhost:25", fessEnv.getMailSmtpServerMainHostAndPort());

        // Test getMailSubjectTestPrefix
        assertEquals("[Test]", fessEnv.getMailSubjectTestPrefix());

        // Test getMailReturnPath
        assertEquals("root@localhost", fessEnv.getMailReturnPath());
    }

    // Test custom properties