Both parts are limited to 7-bit ASCII or a subset thereof. Emojis have to be in punycode. You could theoretically use UTF-7 for the local-part but nothing supports it in practice.
That's the spec, for sure! The domains are converted to punycode, but the usernames? I have no idea what happens to them. My tests showed some providers delivered them, some didn't. I'm sure there are a bunch of people out there using accented characters and CJK etc in their usernames and wondering why it's not going so well...