It is important to understand that Mailster is a part of Joomla, which is a PHP-based web application.
That means: it can not act/run without being triggered, and it can not run forever when triggered. This is a technical limitation coming from PHP, not from Mailster or Joomla. Triggering means that somebody accesses the site. During the page load, Mailster is handling the jobs to do (mail retrieving and sending).
Thus, emails can only be sent/retrieved when somebody is browsing your site, otherwise, the delivery is delayed or never done. As your site might not be browsed every few minutes 24/7, we recommend that you use a cronjob that opens the site periodically.
There are three options available for cronjobs: