From the StackCP control panel, create a "Scheduled Task" (aka cron job) to check for any mailshots waiting to be sent, and process any that are found. Assuming that the mailMan Plugin has been installed and activated, a task must be created to run every 15 minutes, with the format similar to this

/usr/bin/php83 /home/sites/4a/3/376204d07d/public_html/www03/cli/joomla.php ra_mailman:sendemails

Note that the version of PHP that is required, and the prefix to the joomla directory will differ from the above example.

TIP. From the cPanel panel, look in the list of "Scheduled Tasks" for an entry that does the daily backup. This will be formatted like the example above, so copy and paste the entry into a new task and change "akeeba:backup" to "ra_mailman:sendemails".