Installation - How to install Newsletter Add-on for WordPress Download Manager.
Download and install WPDM Newsletter plugin through WordPress admin panel.
If you want to install this plugin via FTP client like FileZilla, unzip this plugin to your computer. Upload “wpdm-newsletter” directory to your sites “/wp-content/plugins/” directory.
Now search for “WPDM - Newsletters” in your WordPress admin panel by going Plugins -> Installed Plugins.
Activate “WPDM - Newsletters” plugin by clicking “Activate” link which is below the title “WPDM - Newsletters”.
If you successfully Activate this plugin, you will see a message “Plugin activated.” at the top of the page. You will also notice a new menu named “Campaign” in your WordPress admin panel.
The plugin use wp cron to check newsletter schedule and send them, but wp cron is not very reliable, so, if you don't want to rely on wp cron, you can setup an cron job from your hosting control panel for the url
http://your-site-name.com/?cron_handle=1 with call interval 5 to 30 mins