Sending Replies and Forwarding Emails with EmailEngine

TL;DR
Add a reference object to your /v1/account/:id/submit payload, set action to "reply" , "replyAll" or "forward", and EmailEngine fills in every header so the message threads exactly like in a desktop email client.

Why it matters

Writing a raw RFC 822 email that threads correctly is deceptively hard—In‑Reply‑To, References, prefixes like Re:/Fwd:, attachment handling, the IMAP \Answered flag… and that’s before you juggle every provider’s SMTP quirks. EmailEngine eliminates that boilerplate so you can reply or forward with one POST request.

Step‑by‑step

1. Choose the message

You’ll need the opaque message identifier (AAAADQAABl0 in the examples). Fetch it via the messages list or use the value included in any webhook EmailEngine sends when syncing mail.

2. Build the reply payload

$ curl -XPOST "https://emailengine.example.com/v1/account/example/submit" \
  -H "Authorization: Bearer <token>" \
  -H "Content-Type: application/json" \
  -d '{
    "reference": {
      "message": "AAAADQAABl0",
      "action": "reply",
      "inline": true
    },
    "html": "<p>Hello from myself!</p>"
}'

Response

{
  "response": "Queued for delivery",
  "messageId": "<[email protected]>",
  "queueId": "24279fb3e0dff64e",
  "sendAt": "2025-05-14T10:02:27.135Z"
}

✅ EmailEngine auto‑sets from, to, subject, In‑Reply‑To, References, and marks the original message with the IMAP \Answered flag.

3. Build the forward payload

$ curl -XPOST "https://emailengine.example.com/v1/account/example/submit" \
  -H "Authorization: Bearer <token>" \
  -H "Content-Type: application/json" \
  -d '{
    "reference": {
      "message": "AAAADQAABl0",
      "action": "forward",
      "inline": true,
      "forwardAttachments": true
    },
    "to": {
      "name": "Andris Reinman",
      "address": "[email protected]"
    },
    "html": "<p>FYI — see below</p>"
}'

✅ EmailEngine adds Fwd: to the subject, prepends the original message with a header block, copies attachments when forwardAttachments is true, and still marks the source message as \Answered.

Common pitfalls

⚠️ Missing to on forward – Unlike replies, forwards require you to set the to field. Omit it and EmailEngine returns 400 Bad Request.

💡 Huge attachments – EmailEngine streams attachments from IMAP to SMTP. If the total size breaches the mailbox’s send limit, the SMTP server will bounce the message. Use forwardAttachments:false or filter the attachments you copy.

⏳ Timeouts on slow SMTP hosts – Some PaaS providers kill idle sockets. Increase smtpTimeout, scale your dynos, or move EmailEngine off the constrained host.