Receiving

Reply to received emails

Reply in-thread by setting In-Reply-To (and References) to the received message_id when sending via POST /emails.

Email clients thread messages using the Message-ID header. To reply to a received email so it lands in the same conversation, send a new email via POST /emails with an In-Reply-To header set to the original message's message_id. Start the subject with Re: so clients group the replies together.

Get the message ID

The message_id is included in the email.received webhook payload:

{
  "type": "email.received",
  "data": {
    "email_id": "56761188-7520-42d8-8898-ff6fc54ce618",
    "message_id": "<111-222-333@email.example.com>",
    "subject": "Sending this example",
    "from": "Acme <onboarding@mailblastr.dev>",
    "to": ["delivered@mailblastr.dev"]
  }
}

Use event.data.message_id as the In-Reply-To header value when sending your reply.

Send a reply in thread

Set the In-Reply-To header to the received message_id and prefix the subject with Re:. See Custom headers for how MailBlastr writes custom MIME headers.

import { MailBlastr } from 'mailblastr';

const mb = new MailBlastr('mb_xxxxxxxxx');

const { data, error } = await mb.emails.send({
  "from": "Acme <hello@yourdomain.com>",
  "to": ["delivered@example.com"],
  "subject": "Re: Sending this example",
  "html": "<p>Thanks for your email!</p>",
  "headers": {
    "In-Reply-To": "<111-222-333@email.example.com>"
  }
});
console.log({ data, error });

Reply headers

In-Reply-Tostringrequired

The message_id of the message you are replying to (e.g. <111-222-333@email.example.com>).

Referencesstringoptional

Space-separated list of every prior message_id in the thread, ending with the one you are replying to. Used for deep threading across multiple replies.

Replying multiple times in a thread

When you reply more than once in the same conversation, also set the References header to all previous message_ids separated by spaces, ending with the current one. This helps clients keep the thread structure correct.

import { MailBlastr } from 'mailblastr';

const mb = new MailBlastr('mb_xxxxxxxxx');

const { data, error } = await mb.emails.send({
  "from": "Acme <hello@yourdomain.com>",
  "to": ["delivered@example.com"],
  "subject": "Re: Sending this example",
  "html": "<p>Thanks for your email!</p>",
  "headers": {
    "In-Reply-To": "<111-222-333@email.example.com>",
    "References": "<msg_id1@domain.com> <msg_id2@domain.com> <111-222-333@email.example.com>"
  }
});
console.log({ data, error });

The reply's from must be on a verified sending domain. The reply is an ordinary outbound send via POST /emails; the In-Reply-To / References headers are what place it in the existing thread.