The same is true of Chinese, Japanese, and other nationalities that do not use Latin scripts, but also applies to users from non-English-speaking European countries whose desired addresses might contain diacritics (e.g. André or Płużyna).
The 2012 standards RFC 6532 and RFC 6531 allow the inclusion of Unicode characters in a header content using UTF-8 encoding, and their transmission via SMTP—but in practice support is only slowly rolling out.
UTF-8 parts, known as U-Labels, are transformed into A-Labels via an ad-hoc method called IDNA.
[2][6] This proposal was deemed too cumbersome; the meaning of the left hand side part of an email address is local to the target server, and so there is no way to check whether xn--something is a valid user name, used in some domain.
The document set also includes discussion of key assumptions and issues in deploying fully internationalized email.