Recently I had some problems when sending e-mails with simplenews newsletter module for Drupal. While this is a very good module, sometimes made Drupal cron to fail and stuck. I also wanted full control on sending mails and to avoid some of the limitations of Drupal cron timings. Since I didn't have the time to enhance the simplenews module I did this script to be sure that my mails are sent. This script needs simplenews newsletter module and Drupal in order to work. It has been tested with Drupal 7 (but should work on any version) and simplenews 7.
The script works out of drupal and speaks to the database so its better for performance and mail control. You can set the time step and the mails it sends every time based on your server sending limits. At the end logs any bad mails. For me it is the solution for sending my newsltters.
You can learn more and get the script from
http://tecorange.com/content/mail-send-addon-drupal-simplenews-newsletter