"this email could not be delivered,"
Where did you get that error message? In your own mail client, or from a website? If it was an automatic response from your isp or the other isp, then something was wrong that itch has no control over. Like you having a typo in the adress or a temporary failure of some kind. You should read the error again for further details. Because usually the systems will tell you exactly why something could not be delivered. Maybe your isp is on a blackhole list or some other crap.