While Unicode characters have been supported in mail bodies and many header fields for a long time (using MIME), the local part of a mailbox (the part to the left of the '@') was restricted to ASCII characters until the SMTPUTF8 extension, published as a proposed standard in February 2012.1
The local part of an email address is very often some form of a person's name. There is no accepted way to represent many (most?) people's names using only ASCII charachters.
If email is to be considered a universal Internet standard, more than just ASCII will be required.
Many major email service providers (Google, Microsoft, etc.) support this extension. Some FLOSS MTA software programs (recent versions of Postfix and EXIM) support this extension.
1. [There was UTF8SMTP, an experimental extension published in 2008] ↩