hook for maintenance mode
As @Clive said you can use hook_form_alter() or hook_form_FORM_ID_alter() and add extra submit handler to send email notifications.

function YOURMODULE_form_system_site_maintenance_mode_alter(&amp;$form, &amp;$form_state, $form_id) {
  $form['#submit'][] = 'email_system_settings_form_submit';
}

function email_system_settings_form_submit($form, &amp;$form_state) {
  // Add your email code here
}