Testing instructions:

1. Enter wrong email settings in global config
2. Register a user in frontend assuming that user activation is required.
3. See COM_USERS_REGISTRATION_SEND_MAIL_FAILED message
4. Load /administrator/index.php?option=com_messages - no messages.
5. Check #__messages table - see empty user_id_from and user_id_to data.

Issue origin:

UsersModelRegistration loads users to send messages to via

->select($db->quoteName(array('name', 'email', 'sendEmail', 'id'))) 
...
$sendEmail = $db->loadColumn();

Next they are iterated via

foreach ($sendEmail as $userid)

Hence email address to stored and converted to 0

Fix:

Change
->select($db->quoteName(array('name', 'email', 'sendEmail', 'id')))
to
->select($db->quoteName('id'))