I copy a before used template and make some style adjustments. When I try to send test mails from this copied template and try to send a test mail, MailJet Webinterface says, that test mail have successfully been send - however, mail never receives.

Your technician told me, that this error occurs, because there is a bug in the template.

It would be great to do NOT get the false "success" mail when sending test mail. Instead it would be good to see, why the mail is reproduceable NOT send.