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 instructions those documents gave to IANA were not consistent with one another. As a result the registries no longer present a reliable picture of the relative status or usage of the fields they contain: fields of equal standing carry different status values, foundational fields appear less classified than minor ones, the "trace" property is recorded for only a fraction of the fields whose defining documents describe them as trace fields, and the reference column frequently points at a later registration document rather than at the document that actually defines or updates the field. This reduces the value of the metadata to the point where a reader cannot use it to reason about a field with confidence. 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 correcting each registry entry. Every recommended change, and every deliberate decision to leave a non-obvious entry unchanged, is justified by reference to the clear intent of the authors of the source documents in which the field was defined or modified.