def put_dict_value(data, address, value):
    """Accepts a dictionary and an "address" (a list representing a nested dict value's key)
    and sets the value at that "address".
        >>> d = {'mailserver_users': [{...}, {...}], ...}
        >>> a = ['mailserver_users', 1, 'newkey']
        >>> put_dict_value(d, a, 'newval')
        {..., 'mailserver_users': [{...}, {'newkey': 'newval', ...}]}
    """
    for key in address[:-1]:
        data = data[key]
    data[address[-1]] = value
    return data
