In the beginning, email was always in English. The content, the headers, the addresses, the protocol — all American English, or fairly close. But the rest of the world uses email, too, and while English remains the lingua franca of the internet, it is not universal — nor does it need to be.
Expanding beyond the initial English alphabet and characters was, and remains, a challenge for email systems. MIME appeared in the early 1990s, expanding the range of character sets available in message bodies — but only the bodies, not the headers or addresses. An SMTP extension called 8BITMIME eased the exchange of these messages across compatible SMTP servers. And now, the IETF’s Email Address Internationalization (EAI) working group has devised a way for international characters to traverse the entire email sending delivery process.
Some of the details are still being worked out, but this is the biggest change to email since MIME itself. My friend and colleague John Levine, who has been following the internationalization discussions, explains further in a series of articles on CircleID: first, second, and third.
If you work on email technology, this is required reading.