QMessageFolderFilter::byParentFolderId() and QMessageFolderFilter::byAncestorFolderIds() returns invalid QMessageFolderFilter object, QMessageFolderFilter::isSupported () returns false.

For this reason QMessageManager::queryFolders() returns always all folders. It is not possible to list subfolders.

It seems that mentioned functionality was not implemented. Please check code in qmessagefolderfilter_symbian.cpp:

QMessageFolderFilter QMessageFolderFilter::byParentFolderId(const QMessageFolderId &id, QMessageDataComparator::EqualityComparator cmp)
{
    Q_UNUSED(id)
    Q_UNUSED(cmp)
    QMessageFolderFilter result;
    result.d_ptr->_valid = false;
    return result;
}