you may also need to adjust the wait time between each email. it may be that the sending an email each second is clogging their SMTP Server and causing the script stop running.
basically there is a server limitation in place that is causing the problem and you will need to adjust the time between email to get around their limitations.
some servers limit the number of emails you can send in a day, it may be that you have reached that limit and the server will not allow to send more emails till tomorrow.
With large email lists, it is sometimes best to stage the audiences so that you are sending to smaller audiences to send the mailing in chunks.