It would be awesome if you could send a test email from the mass email page before hitting the send button to send it to everyone.
The default would be to send it to the logged in admin user who is setting up the mass email, but it should also be able to send the test email to any email address that the admin enters, as well as allowing them to enter a prefix message to the email, e.g. "hey Bob, this is the email I'm going to send tomorrow, please let me know if you see any typos!"