Display additional fields in the Inbox?
/**
 * Implements hook_form_alter().
 */
function modulename_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form_id == 'privatemsg_list') {
    $form['updated']['list']['#header']['bar_column_field']['data'] = 'Bar Column';
    $form['updated']['list']['#header']['bar_column_field']['field'] = 'bar_column_field';
    $form['updated']['list']['#header']['bar_column_field']['class'][] = 'privatemsg-header-bar-column';
    $form['updated']['list']['#pre_render'][] = '_pre_render_bar_column';
  }

/**
 * Custom #pre_render function.
 */
function _pre_render_bar_column($tableselect) {  
  $i = 0;
  foreach ($tableselect['#options'] as $id =&gt; $options) {
    $data = 'Bar data in the table ' . $i++ . '';
    $tableselect['#options'][$id]['bar_column_field'] = array('data' =&gt; $data, 'class' =&gt; array('privatemsg-list-bar'));
  }
  return $tableselect;
}


You can change the $data variable with any field's value, after performing a query or something, and placing your actual data to the new column.
A field could be attached to messaging (admin/config/messaging/privatemsg/fields) or to any other available entity.