Apple ID Rescue Email: 'Enter a valid email address.'

To resolve this problem, I ended up linking a Gmail address as an Apple Rescue e-mail. I would have preferred to use something within my own domain, as it's easier to manage and re-route, but such are Apple's restrictions.

It is ironic, in a sense. The only way I could get an Apple account working properly is to use a Google product.

Apple's authentication process seems quaint. Like journeying back to 2005, they try to push you into phone number based 2FA. Bafflingly, they somehow think a phone number is secure enough, yet they don't offer options like open-source TOTP, and you can't link a FIDO2 hardware key to your account.

It's a good thing my Gmail account has such protections, as I wouldn't want the password recovery mechanism to be exploited. I do not like the idea of living in fear of losing my identity to a SIM-swap attack. Thus, I never linked a phone number to Apple for 2FA purposes.

Perhaps Apple don't give much consideration to people who only dip a toe into their ecosystem. Not every desires to be dependent on a iPhone. I can only describe my Apple ID authentication experience in 1 word: Confounding.