Sending multiple emails in the same thread with EmailEngine

TL;DR
Call POST /v1/account/:id/submit with your own messageId and a growing references header to force every follow‑up into the same email thread.

Why it matters

Email clients rely on the RFC 5322 Message‑ID and References headers—never on SMTP commands—to decide which messages belong together. If you let EmailEngine autogenerate those values, your perfectly timed sequence may scatter across the inbox. By controlling them yourself, every follow‑up lands exactly where the user expects.

Step‑by‑step

1. Send the initial message

$ curl -XPOST "http://127.0.0.1:3000/v1/account/demo/submit" \
  -H "Authorization: Bearer $EE_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
    "from": { "address": "sender@example.com" },
    "to":   { "address": "recipient@example.com" },
    "subject": "Test message thread",
    "html": "<p>First message in thread!</p>",
    "messageId": "<56b3c6d2-f7c0-4272-8beb-e25fdb7c19f1@example.com>"
  }'

Save the messageId value—you’ll need it for every reply.

2. Add the first follow‑up

$ curl -XPOST "http://127.0.0.1:3000/v1/account/demo/submit" \
  -H "Authorization: Bearer $EE_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
    "from": { "address": "sender@example.com" },
    "to":   { "address": "recipient@example.com" },
    "subject": "Test message thread",
    "html": "<p>Second message in thread!</p>",
    "messageId": "<77a7c383-cc1a-44c6-9866-96b2873e3322@example.com>",
    "headers": {
      "references": "<56b3c6d2-f7c0-4272-8beb-e25fdb7c19f1@example.com>"
    }
  }'

3. Keep extending references

Each subsequent call appends the current message’s ID:

"headers": {
  "references": "<56b3c6d2-f7c0-4272-8beb-e25fdb7c19f1@example.com> <77a7c383-cc1a-44c6-9866-96b2873e3322@example.com>"
}

Common pitfalls

⚠️ Missing angle brackets – Wrap every ID in < > or some clients ignore the header.

💡 Subject drift – Changing the subject (beyond adding Re:) breaks the thread despite perfect headers.

🚧 Gmail limit – Gmail reads only the last 20 References entries. If your sequence is longer, drop the oldest IDs.

🗄️ ID storage – Persist every generated messageId so you can rebuild the references header later; EmailEngine doesn’t store that list for you.