Emoji domains can be registered using punycode, and you're right that it's up to the TLD whether they're allowed or not.

For example: http://xn--yt8h.la/%F0%9F%90%B6

📙.la is encoded using punycode to http://xn--yt8h.la/

🐶 is URL-encoded to %F0%9F%90%B6

Giving the 'true' URL http://xn--yt8h.la/%F0%9F%90%B6 which then redirects to https://emojipedia.org/dog-face

Emails should generally use @xn--yt8h.la instead of @📙.la for maximum compatibility. I'm not sure if the email spec allows punycode.