It's not an iCloud address. The Sign in with Apple Hide My Email (<random>@privaterelay.appleid.com
) is different from the iCloud+ Hide My Email (<human-readable-random>@icloud.com
), and there are more restrictions on the Sign in with Apple ones. The hidden email is associated with an app/website (and thus developer), and only email addresses associated with the developer are allowed to send to that hidden email. This sounds fine, except lots of sites have multiple domains or use mailing services and don't configure things correctly, so sometimes you'll never receive mail from them. Or you have to send email to the site from your registered email (as proof of account ownership), but you can't send mail from these addresses. Honestly, having used Sign in with Apple before, I can't recommend it to anyone who has iCloud+ (or another email alias service) because it's a huge hassle if you ever run into the aforementioned issues.