Email Subscribers & Newsletters <= 5.9.10 - Missing Authentication to Unauthenticated Mailing Queue Trigger

The Icegram Express - Email Subscribers, Newsletters and Marketing Automation Plugin for WordPress is vulnerable to Authorization in versions up to, and including, 5.9.10. This is due to the plugin not properly verifying that a user is authorized to perform an action in the `trigger_mailing_queue_sending` function. This makes it possible for unauthenticated attackers to force immediate email sending, bypass the schedule, increase server load, and change plugin state (e.g., last-cron-hit), enabling abuse or DoS-like effects.