https://wordpress.org/support/topic/http-post-requests-every-20-40-seconds-from-mailpoet-cron/ 2021
Newsletter task scheduler (cron)
1. Visitors to your website (recommended)
2. MailPoet's own script. Doesn't work with these hosts.
3. Server side cron (Linux cron)
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1. WordPress built-in cron (recommended)
2. Visitors to your website
3. Server side cron (Linux cron)
MailPoet's own script
This is no longer available for 3.89.0 version or higher.
https://ja.wordpress.org/plugins/mailpoet/#developers
> 3.94.0 – 2022-08-08
> Added: new WordPress cron task scheduler method;
> 3.89.0 – 2022-05-09
> Updated: Removed MailPoet Task Scheduler;
