Hello,
I have been developing freewares and sharewares for many years. Since I have many many customers, I developed a similar application (no installation is required) two years ago and have been using this to send bulk emails to all my customers.
You can try this application if you like. This application allow you to set the email sending interval (e.g. 30 seconds, 45 seconds, 60 seconds). The application is fully automatic.
Thanks and best regards
Alex