Many years ago, I proposed a "Jabber-ID" email header so that you could advertise your Jabber/XMPP address in the email messages you send. Somehow it got caught up in the IETF politics of the time and was never standardized. Yet an email header doesn't need to be defined in a standards-track RFC in order to be added to IANA's provisional registry for email header field names (any old specification will do). Although draft-saintandre-jabberid existed in a lapsed state for years, it seemed better to make the document an informational RFC through the independent stream because RFCs are more stable and more referenced than stale Internet-Drafts. Thus RFC 7259. Enjoy!
Peter Saint-Andre > Journal