[Brooooooklyn]

Fixes #665

What

sendEmail gets a typed threading option, so a reply sent later (from a queue job, after the inbound handler returned) stays in the same thread.

await sendEmail({
  from: 'support@acme.dev',
  to: ticket.from,
  subject: `Re: ${ticket.subject}`,
  text: 'Fixed.',
  threading: { inReplyTo: ticket.messageId, references: `${ticket.references} ${ticket.messageId}` },
});

sendEmail({ threading })          replyEmail(message)
        │                                 │
        └──────► normalizeThreading ◄─────┘   (one channel, same rules)
                        │
          threading.ts: threadingId / threadingReferences
                        │
                    buildMime ──► In-Reply-To / References
headers: { 'In-Reply-To' | 'References' } ──► dropped (still reserved)

Answers to the issue's questions

#	Question	Answer
1	Naming	Keep the threading object. It already exists in normalizeEmail.
2	Validation	Same rules as replyEmail: an id with a control character or too long for one line is left out; references keeps the newest ids within 8 KiB and falls back to inReplyTo; references defaults to inReplyTo. The email still sends. Only a non-string value returns MIME_ERROR.
3	Proxy	No change. The proxy forwards the MIME bytes as they are, and already counts these as built-in headers.
4	Spoofing	Does not apply. From is still pinned, and the caller only threads its own outbound message. headers still can't set the two names, so there is one checked path.

Also fixed

The line-length check (fitsHeaderLine) now follows foldHeader's quote rules. Before, a stray " in an id passed the check but could not be folded, so the send failed with MIME_ERROR. This also covers reply subjects, which share the check.

Changes

• runtime/email/threading.ts (new): shared id checks moved out of mime.ts / inbound.ts.
• mime.ts: normalizeThreading checks the option; inReplyTo is trimmed of SP/HTAB only, matching Headers.
• types.ts: SendEmailOptions.threading with JSDoc.
• Docs: options row and a "Reply later" section in docs/guide/email/sending.md; a link from receiving.md.
• Tests: email-mime.test.ts, email-send.test.ts.

Verification

• vp test run on the email unit tests: 432 passed.
• vp run test --base origin/main: 12,093 passed.
• Typecheck, lint, fmt clean.

🤖 Generated with Claude Code

[chatgpt-codex-connector]

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-11-02T15:21:11.099766Z	962bc55	PR opened

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