Automatic emails are not sent – preview emails are

I am new to WordPress and trying to activate the newsletter in MailPoet to automatically send mails when there is new content. As mail server I use SendGrid. The preview mails work perfectly – in the Send With… tab as well as sending the preview newsletter when creating it. But when I create a new post, nothing happens.
I enabled the cron and I use an external service for triggering the written URL every minute.
Can anybody help me with this problem? I tried subscribe2 as well but there I have the same problem.

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