I almost caused a mailout like this as well when I just kept adding recipients in a loop without ever clearing the recipient list again. (Using SwiftMailer, that is.)

A 'fake' SMTP server that just saved the e-mails to the file system as plain textfiles helped me to find the error before I actually sent the mails out. I am not entirely sure but I think it was this project: http://sourceforge.net/projects/fakemail/