What is the Newsletter Task Scheduler?
When you hit the send button, MailPoet adds your email to what's called a "sending queue". The task scheduler monitors the sending queue and dispatches emails in batches according to their priority and the sending frequency (X amount of emails every Y minutes) set under MailPoet > Send with... option. The task scheduler is also responsible for sending ensuring that Post Notifications and Welcome Emails are processed and dispatched on time.
There are 2 ways to trigger (start) the task scheduler:
a) Visitors to your website: every visit (page load) to your website starts the task scheduler that temporarily runs in the background until all emails are processed. This is the default method but it can be unreliable on sites with low or no traffic.
b) MailPoet's own script: the task scheduler runs as a non-stop background process and does not depend on site visitors. However, it uses minimal server resources on a constant basis and that may result in your account being disabled by some hosting providers who limit CPU/RAM utilization.