Newsletters: Set up a Server Cron Job
WordPress Newsletter plugin: Configuring a Server Cron Job
Use Server Cron For WordPress Globally
Consider relaying your WordPress cron job globally with a server cron job to provide accuracy, reliability and better performance.
The WordPress Newsletter plugin uses the WordPress cron job for all of it’s schedules internally so as long as your WordPress cron runs and works, the tasks will run as they should.
Read the Replace WordPress Cron with a Real Cron Job for Site Speed article.