2025-08-25 Expert Review Mark Baker (primary), Murray Kucherawy (backup) specified that no new header fields be registered that begin with "Downgraded-". That restriction is now lifted, per . HTTP field name registrations have been moved to per . Accept-Language mail Also-Control netnews obsoleted Alternate-Recipient mail Approved netnews standard ARC-Authentication-Results mail experimental ARC-Message-Signature mail experimental ARC-Seal mail experimental Archive netnews standard Archived-At mail standard Archived-At netnews standard Article-Names netnews obsoleted Article-Updates netnews obsoleted Authentication-Results mail standard Auto-Submitted mail standard RFC3834 section 5 Autoforwarded mail Autosubmitted mail Base MIME obsoleted RFC2068 Section 14.11 Bcc mail standard no RFC-ietf-emailcore-rfc5322bis-12, 3.6.3 Body none reserved Cancel-Key netnews standard Cancel-Lock netnews standard Cc mail standard no RFC-ietf-emailcore-rfc5322bis-12, 3.6.3 Comments mail standard no RFC-ietf-emailcore-rfc5322bis-12, 3.6.5 Comments netnews standard Content-Alternative MIME Content-Base MIME obsoleted Content-Description MIME Content-Disposition MIME Content-Duration MIME Content-features MIME Content-ID MIME Content-Identifier mail Content-Language MIME Content-Location MIME Content-MD5 MIME Content-Return mail Content-Transfer-Encoding MIME Content-Translation-Type MIME standard Content-Type MIME Control netnews standard Conversion mail Conversion-With-Loss mail DL-Expansion-History mail Date mail standard no RFC-ietf-emailcore-rfc5322bis-12, 3.6.1 Date netnews standard Date-Received netnews obsoleted Deferred-Delivery mail Delivery-Date mail Discarded-X400-IPMS-Extensions mail Discarded-X400-MTS-Extensions mail Disclose-Recipients mail Disposition-Notification-Options mail Disposition-Notification-To mail Distribution netnews standard DKIM-Signature mail standard Downgraded-Bcc mail obsoleted Downgraded-Cc mail obsoleted Downgraded-Disposition-Notification-To mail obsoleted Downgraded-Final-Recipient mail standard RFC6857 Section 3.1.10 Downgraded-From mail obsoleted RFC6857 Section 3.1.10 Downgraded-In-Reply-To mail standard RFC6857 Section 3.1.10 Downgraded-Mail-From mail obsoleted RFC6857 Section 3.1.10 Downgraded-Message-Id mail standard RFC6857 Section 3.1.10 Downgraded-Original-Recipient mail standard RFC6857 Section 3.1.10 Downgraded-Rcpt-To mail obsoleted Downgraded-References mail standard RFC6857 Section 3.1.10 Downgraded-Reply-To mail obsoleted Downgraded-Resent-Bcc mail obsoleted Downgraded-Resent-Cc mail obsoleted Downgraded-Resent-From mail obsoleted Downgraded-Resent-Reply-To mail obsoleted Downgraded-Resent-Sender mail obsoleted Downgraded-Resent-To mail obsoleted Downgraded-Return-Path mail obsoleted Downgraded-Sender mail obsoleted Downgraded-To mail obsoleted Encoding mail Encrypted mail Expires mail Expires netnews standard Expiry-Date mail Followup-To netnews standard From mail standard no RFC-ietf-emailcore-rfc5322bis-12, 3.6.2 From netnews standard Generate-Delivery-Report mail HP-Outer mail standard RFC9788 Section 2.2.1 Importance mail In-Reply-To mail standard no RFC-ietf-emailcore-rfc5322bis-12, 3.6.4 Incomplete-Copy mail Injection-Date netnews standard Injection-Info netnews standard Keywords mail standard no RFC-ietf-emailcore-rfc5322bis-12, 3.6.5 Keywords netnews standard Language mail Latest-Delivery-Time mail Lines netnews deprecated List-Archive mail List-Help mail List-ID mail List-Owner mail List-Post mail List-Subscribe mail perm/list-unsubscribe List-Unsubscribe mail List-Unsubscribe-Post mail standard Message-Context mail Message-ID mail standard no RFC-ietf-emailcore-rfc5322bis-12, 3.6.4 Message-ID netnews standard Message-Type mail MIME-Version MIME MMHS-Exempted-Address mail ACP123 Appendix A1.1 and Appendix B.105 MMHS-Extended-Authorisation-Info mail ACP123 Appendix A1.2 and Appendix B.106 MMHS-Subject-Indicator-Codes mail ACP123 Appendix A1.3 and Appendix B.107 MMHS-Handling-Instructions mail ACP123 Appendix A1.4 and Appendix B.108 MMHS-Message-Instructions mail ACP123 Appendix A1.5 and Appendix B.109 MMHS-Codress-Message-Indicator mail ACP123 Appendix A1.6 and Appendix B.110 MMHS-Originator-Reference mail ACP123 Appendix A1.7 and Appendix B.111 MMHS-Primary-Precedence mail ACP123 Appendix A1.8 and Appendix B.101 MMHS-Copy-Precedence mail ACP123 Appendix A1.9 and Appendix B.102 MMHS-Message-Type mail ACP123 Appendix A1.10 and Appendix B.103 MMHS-Other-Recipients-Indicator-To mail ACP123 Appendix A1.12 and Appendix B.113 MMHS-Other-Recipients-Indicator-CC mail ACP123 Appendix A1.12 and Appendix B.113 MMHS-Acp127-Message-Identifier mail ACP123 Appendix A1.14 and Appendix B.116 MMHS-Originator-PLAD mail ACP123 Appendix A1.15 and Appendix B.117 MT-Priority mail standard Newsgroups netnews standard NNTP-Posting-Date netnews obsoleted NNTP-Posting-Host netnews obsoleted Obsoletes mail Organization mail informational Organization netnews standard Original-Encoded-Information-Types mail Original-From mail standard Original-Message-ID mail perm/original-recipient Original-Recipient mail standard Original-Sender netnews standard Originator-Return-Address mail Original-Subject mail standard Path netnews standard PICS-Label mail Posting-Version netnews obsoleted Prevent-NonDelivery-Report mail Priority mail Received mail standard yes RFC-ietf-emailcore-rfc5322bis-12, 3.6.7 Received-SPF mail standard References mail standard no RFC-ietf-emailcore-rfc5322bis-12, 3.6.4 References netnews standard Relay-Version netnews obsoleted Reply-By mail Reply-To mail standard no RFC-ietf-emailcore-rfc5322bis-12, 3.6.2 Reply-To netnews standard Require-Recipient-Valid-Since mail standard Resent-Bcc mail standard no RFC-ietf-emailcore-rfc5322bis-12, 3.6.6 Resent-Cc mail standard no RFC-ietf-emailcore-rfc5322bis-12, 3.6.6 Resent-Date mail standard no RFC-ietf-emailcore-rfc5322bis-12, 3.6.6 Resent-From mail standard no RFC-ietf-emailcore-rfc5322bis-12, 3.6.6 Resent-Message-ID mail standard no RFC-ietf-emailcore-rfc5322bis-12, 3.6.6 Resent-Reply-To mail obsoleted no RFC-ietf-emailcore-rfc5322bis-12, 4.5.6 Resent-Sender mail standard no RFC-ietf-emailcore-rfc5322bis-12, 3.6.6 Resent-To mail standard no RFC-ietf-emailcore-rfc5322bis-12, 3.6.6 Return-Path mail standard yes RFC-ietf-emailcore-rfc5322bis-12, 3.6.7 See-Also netnews obsoleted Sender mail standard no RFC-ietf-emailcore-rfc5322bis-12, 3.6.2 Sender netnews standard Sensitivity mail Solicitation mail Subject mail standard no RFC-ietf-emailcore-rfc5322bis-12, 3.6.5 Subject netnews standard Summary netnews standard Supersedes mail Supersedes netnews standard TLS-Report-Domain mail standard TLS-Report-Submitter mail standard TLS-Required mail standard To mail standard no RFC-ietf-emailcore-rfc5322bis-12, 3.6.3 User-Agent netnews standard VBR-Info mail standard X400-Content-Identifier mail X400-Content-Return mail X400-Content-Type mail X400-MTS-Identifier mail X400-Originator mail X400-Received mail X400-Recipients mail X400-Trace mail Xref netnews standard Expert Review Mark Baker (primary), Murray Kucherawy (backup) Registration of a Provisional Message Header Field does not of itself imply any kind of endorsement by the IETF, IANA or any other body. HTTP field name registrations have been moved to per . prov/apparently-to Apparently-To mail Author mail CFBL-Address mail CFBL-Feedback-ID mail Delivered-To mail EDIINT-Features mail Eesst-Version mail prov/errors-to Errors-To mail prov/face Face mail prov/face Face netnews Form-Sub mail prov/jabber-id Jabber-ID mail prov/jabber-id Jabber-ID netnews MMHS-Authorizing-Users mail Privicon mail SIO-Label mail SIO-Label-History mail prov/wrong-recipient Wrong-Recipient mail prov/x-archived-at X-Archived-At mail deprecated prov/x-archived-at X-Archived-At netnews deprecated prov/x-face X-Face mail prov/x-face X-Face netnews X-Mittente mail prov/x-pgp-sig X-PGP-Sig netnews X-Ricevuta mail X-Riferimento-Message-ID mail X-TipoRicevuta mail X-Trasporto mail X-VerificaSicurezza mail Specification Required Nik Tomkinson original Content in the original language human Content that has been translated by a human translator or a human has checked and corrected an automated translation automated Content that has been translated by an electronic agent without proofreading or subsequent correction