Disabled checkbox "for automatically sign up for email notifications"

The checkbox is allways disabled.

Reason:

if($account != $user) {
    $form['message_subscribe_email']['#disabled'] = TRUE;
  }

is always true, even when visiting the own profile:

Solution:

if($account->uid != $user->uid) {
    $form['message_subscribe_email']['#disabled'] = TRUE;
  }

does the job