The queue API allows placing items in a queue and processing them later. If you have a task that must be done regularly and it takes more time to complete then Cron queue might be the better solution.
In Drupal 8, hook_cron_queue_info() has been replaced by QueueWorker Plugin. Define the queue worker by creating a QueueWorker folder inside the Plugin folder. In the below example, I create a form for sending the email. Emails will be added to the queue by submitting the form. On cron run emails will be dequeued.