---
id: "references"
kind: "datatype"
status: "stable"
is_pii: false
categories: ["internet"]
langs: ["common"]
quality: 90
url: "http://registry.apicrafter.io/datatype/references"
---

# References (Email Header) (`references`)

Value of the References header of an email message (RFC 5322, section 3.6.4): the Message-IDs of the earlier messages of the thread, separated by whitespace. A reply copies the References of the message it answers and appends that message's Message-ID, so mail clients can rebuild a thread even when some of its messages are missing.

- Personal data (PII): no
- Categories: Internet (internet)
- Countries: global
- Languages: Common to most languages (common)
- Classification: identifier

## Detection

Regular expression (Python `re`):

```
^<[^<>@\s]+@[^<>@\s]+>(?:\s+<[^<>@\s]+@[^<>@\s]+>)*$
```

## Examples

- `<1234@local.machine.example> <3456@example.net>` — Second reply of the thread in RFC 5322, appendix A.2
- `<1234@local.machine.example>` — First reply of the thread in RFC 5322, appendix A.2
- `1234@local.machine.example, 3456@example.net` (invalid example) — No angle brackets

## Metacrafter rules

1 rules: 1 match the field name, 0 match values.

- `referencesfield` (field, `common/internet.yaml`)

## Supported by tools

- `metacrafter`

## Translations

- ru: Заголовок письма References: Значение заголовка References: Message-ID предыдущих писем цепочки через пробел (RFC 5322).

## Sources

- wikipedia: <https://en.wikipedia.org/wiki/Message-ID>
- doc: <https://www.rfc-editor.org/rfc/rfc5322.html#section-3.6.4>

## Quality

Score 90/100. Missing: mapping (Wikidata, Schema.org, ...).

JSON: <http://registry.apicrafter.io/api/v1/datatypes/references.json> · Edit: <https://github.com/apicrafter/metacrafter-registry/edit/main/data/datatypes/any/internet/references.yaml>