mail-threading 0.1.3

JWZ email threading (RFC 5256). Reconstruct message threads from References/In-Reply-To headers, with subject fallback for broken clients.
Documentation 
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{
  "name": "duplicate-message-id-first-wins",
  "description": "Duplicate Message-ID input keeps the first message associated with that Message-ID and gives later duplicates unique identities.",
  "spec": {
    "source": "RFC 5256",
    "url": "https://www.rfc-editor.org/rfc/rfc5256",
    "behavior": "the first message with a duplicate Message-ID uses that ID; subsequent duplicates are assigned unique IDs"
  },
  "input": [
    {
      "id": "first",
      "message_id": "<dup@example>",
      "in_reply_to": null,
      "references": [],
      "subject": "Duplicate",
      "date": "2026-05-15T10:00:00Z"
    },
    {
      "id": "second",
      "message_id": "<dup@example>",
      "in_reply_to": null,
      "references": [],
      "subject": "Duplicate replacement",
      "date": "2026-05-15T08:00:00Z"
    },
    {
      "id": "child",
      "message_id": "<dup-child@example>",
      "in_reply_to": "<dup@example>",
      "references": [
        "<dup@example>"
      ],
      "subject": "Re: Duplicate",
      "date": "2026-05-15T09:00:00Z"
    }
  ],
  "expected": {
    "threads": [
      {
        "root": "second",
        "messages": [
          "second"
        ]
      },
      {
        "root": "first",
        "messages": [
          "child",
          "first"
        ]
      }
    ]
  }
}