I am required by my hosting provider to send 4 email per minute. As far as I understood the script sends the email calculating from the maximum numbers per hour, which in our case is 240, every 5 minutes.
No matter how I tried to change the settings and the script it keep on failing to deliver at the per minute rate. Any suggestions?
The cron system of wordpress, used by Newsletter, is not a real cron system and probably cannot run correctly every minute. Newsletter set it to run every 5 minutes so probably your provider does not accept a batch of 20 email at once. Your provider should be more friendly and compute an hourly statistcs not a per minute statistic.
There is no real solution for you: you can consider to use an SMTP service like sendgrid.