The dev/testing data should be anonymized before imported into it to avoid leaking real customer data and also sending emails/notifications to any customers.
There are few tools that are helping do that without much hassle but you could also write a simple module based on
From what I've tested and works:
You can also dump the data stripped for development - without any customer/sales data - from live database with n98-magerun command:
n98 db:dump --strip=@development
What above command does it takes care of all tables that shouldn't be moved to a development environment therefore all email queues etc are also purged.
Another solution is to implement a MailCatcher on your dev machine and configure Magento to use it instead of a production email config.
This could be easily done with LimeSoda_EnvironmentConfiguration but as I've stated earlier - no real customers data should be exposed on dev environment.