Scheduled post is not sent


I scheduled a post to be sent only via email to all members of my blog at Ailola. The post is marked as sent in the Ghost admin panel:

When I open the MailGun panel I cannot find the sent mails anywhere - even not in the log. I re-checked the domain DNS settings, the API keys and also did some test mailings of Ghost posts which are properly sent out via and logged in Ghost.

Now I am wondering why my actual post has not been sent out via MailGun. Is there any way to view the API responses when the scheduled post has been submitted to the MailGun API to figure out on which side the error appears? Where can I find and access this API log in Ghost (if it exists)?

Thank you,