Fastmail The IANA "Message Headers" registries record, for each registered header field, the protocol it belongs to, its status, whether it is a trace field, and the document(s) that specify it. These registries were populated incrementally by many documents over more than two decades, and the metadata that reached the registries is in several respects less complete than the metadata the registering documents supplied. Most notably, the document that performed the single largest bulk registration, RFC 4021, gave IANA an explicit status and an explicit specification document for every one of the roughly ninety fields it registered, and neither was recorded: those entries carry a blank status and cite RFC 4021 itself rather than the specification. Separately, the "Trace" column added to both registries by the ongoing revision of RFC 5322 was deliberately left empty for pre-existing entries, to be filled in later. This document reviews the initial definition of, and every subsequent update to, each registered header field, and gives IANA a single, coherent set of instructions for completing and correcting each registry entry. Every recommended change, and every deliberate decision to leave a non-obvious entry unchanged, is justified by reference to the instructions already given by, or the clear intent of the authors of, the source documents in which the field was defined or modified.