I have a unique use case. I am currently using mailcontrol module to disabled the sending of emails when news users are created. However I got to thinking about this, and I was wondering if there is a way to create a Rule to prevent the email from sending so I wouldn't have to use the mailcontrol module.
Maybe a custom module and lines in the setting.php?
Here is a simple tutorial showing how
$message['send'] = FALSE; can stop it. Of course, you can also use if statments to add conditions.
Note: The developer does mention that you shouldn't use it on production...