SMTPUTF8
The SMTPUTF8 extension allows UTF-8 encoding in mailbox names and header fields. This provides the capability for sending email to internationalized addresses such as Pelé@example.com, δοκιμή@παράδειγμα.δοκιμή, and 测试@测试.测试. This extension is defined in RFC 6531.
Read more about this topic: Extended SMTP