RFC 5337: Internationalized Delivery Status and Disposition Notifications
Extends DSN and MDN formats to support internationalized addresses.
internationalization
RFC 6531: SMTP Extension for Internationalized Email
Defines the SMTPUTF8 SMTP extension that allows UTF-8 in MAIL FROM, RCPT TO, and headers.
RFC 6532: Internationalized Email Headers
Defines UTF-8 encoded headers for internationalized email messages.
RFC 2231: MIME Parameter Value and Encoded Word Extensions
Defines charset and language tagging for MIME parameter values, used in Content-Disposition filenames with non-ASCII characters.
RFC 2047: MIME Part Three: Message Header Extensions for Non-ASCII Text
Defines encoded-word syntax for non-ASCII characters in headers.
