def set_log_format(log_format, server=_DEFAULT_SERVER):
    '''
    Set the active log format for the SMTP virtual server.

    :param str log_format: The log format name.
    :param str server: The SMTP server name.

    :return: A boolean representing whether the change succeeded.
    :rtype: bool

    CLI Example:

    .. code-block:: bash

        salt '*' win_smtp_server.set_log_format 'Microsoft IIS Log File Format'
    '''
    setting = 'LogPluginClsid'
    log_format_types = get_log_format_types()
    format_id = log_format_types.get(log_format, None)

    if not format_id:
        message = ("Invalid log format '{0}' specified. Valid formats:"
                   ' {1}').format(log_format, log_format_types.keys())
        raise SaltInvocationError(message)

    _LOG.debug("Id for '%s' found: %s", log_format, format_id)

    current_log_format = get_log_format(server)

    if log_format == current_log_format:
        _LOG.debug('%s already contains the provided format.', setting)
        return True

    _set_wmi_setting('IIsSmtpServerSetting', setting, format_id, server)

    new_log_format = get_log_format(server)
    ret = log_format == new_log_format

    if ret:
        _LOG.debug("Setting %s configured successfully: %s", setting, log_format)
    else:
        _LOG.error("Unable to configure %s with value: %s", setting, log_format)
    return ret