Hello,
the skipped emails miss a reference to the original post when the error occurs as a response from the SMTP service. E.g., for other reasons, you may click the reason and find what message was affected, but if the SMTP server rejects the message, the admin does not know what message the error is about.
Is there a way to bypass this missing link and figure out what is the failed message? It would be useful to debug since we could have the full message headers. Email support tells me there might be a case of misordered headers that may affect the spam score. But since I do not know what message is rejected I cannot investigate any further.