Prevent email from being sent using hook_mail? I'm trying to implement an access check (a 'do-not-send' list query) just before emails are sent. 
I see how to modify the message, but how do I stop a message?.

HOOK_mail ($key, &amp;$message, $params) {
   if (TRUE) {
      // -- Stop Message From Being Sent Here -- //
   }
}


Is break; adequate?
You can do the access check from hook_mail_alter instead of hook_mail (let it prepare the default message based on parameters, and alter it later) by something like:

function HOOK_mail_alter(&amp;$message) {
  // can add additional check for particular mails here,
  // by checking the value in $message['id']
  if (TRUE) {
    $message['send'] = FALSE;
  }
}


Note: You should clear the cache in order to make the changes effective.