Thank you for reporting this issue. This has been fixed for 7.7.5.
[FIXED] Accidentally sending out a batch of emails when no Send To is entered, has been fixed. Using [Control Panel > Tools & Information > Send Email] if you click the "Send Email" submit button at the bottom without the "Send Email To" address filled, the mailer would send out your message to any addresses stored from the previous Member Search batch-mail session.
