update_mail($key, &$message, $params)
Implements hook_mail().
Constructs the email notification message when the site is out of date.
Parameters
$key: Unique key to indicate what message to build, always 'status_notify'.
$message: Reference to the message array being built.
$params: Array of parameters to indicate what kind of text to include in the message body. This is a keyed array of message type ('core' or 'contrib') as the keys, and the status reason constant (UPDATE_NOT_SECURE, etc)