You only need the php file and a form that will initiate it plus a custom unsubscribe page. The script uses simplenews newsletter database tables, users, newsletter categories etc. The only thing you have to do is to disable the simplenews newsletter spool cron from Drupal or choose not to send them form Simplenews/Drupal interafce in order to avoid having both this tool and Drupal cron sending mails. You can easily dissabel cron if you get the Elysia Cron module for Drupal. Users will continue to subscribe/unsubscribe using simplenews newsletter module and you will create the newsletters the usual way. The script works with multiple newsletter categories and every time processes the latest newsletter that has not been sent. After you sent the newsletter, there is nothing else you need to do. Nothing very fancy but is good for sending thousands of e-mails.
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While simplenews is an excelent modune, this addon gave me confidense when sending my newsletters