https://app.knowbase.one/vault/rfc/docid/7468
RFC6857: Post-Delivery Message Downgrading for Internationalized Email Messages
Internet Engineering Task Force (IETF) K. Fujiwara Request for Comments: 6857 JPRS Category: Standards Track March 2013 ISSN: 2070-1721 Post-Delivery Message Downgrading for Internationalized ...