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

# In-Reply-To (Email Header) (`in_reply_to`)

Value of the In-Reply-To header of an email message (RFC 5322, section 3.6.4): the Message-ID of the message that this one answers, or several of them when it answers more than one. Each identifier is enclosed in angle brackets. Mail clients use it together with the References header to build conversation threads.

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

## Metacrafter rules

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

- `inreplytofield` (field, `common/internet.yaml`)
- `inreplytovalue` (data, `common/internet.yaml`)

## Supported by tools

- `metacrafter`

## Translations

- ru: Заголовок письма In-Reply-To: Значение заголовка In-Reply-To: Message-ID письма, на которое дан ответ (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/in_reply_to.json> · Edit: <https://github.com/apicrafter/metacrafter-registry/edit/main/data/datatypes/any/internet/in_reply_to.yaml>