function mymodule_admin_settings() {

$form = system_settings_form($form);

$form['functions']['mymodule_pm_mail'] = array(
    '#type' => 'textfield',
    '#title' => t('Message'),
    '#value' => $default_value_mail,
  );

  variable_del('mymodule_pm_mail');

  $form['functions']['mymodule_foo'] = array(
    '#type' => 'submit',
    '#value' => t('Submit'),
    '#submit' => array('bla'),
  );


  return $form;
}

function bla() {
  drupal_set_message('Test: ' . $form_state['values']['mymodule_pm_mail']);
}