# SMTP headers and replies

> Which headers Emailit adds, rewrites or removes on SMTP mail, how to read email IDs from the 250 reply, and how to add List-Unsubscribe yourself.

Emailit relays the MIME message you submit over SMTP mostly as it is, but it changes a few headers so the message authenticates and can be tracked. This page lists those changes, explains the `250` reply that carries the new email IDs, and covers the headers you have to add yourself.

## Headers Emailit adds or rewrites

| Header | What Emailit does |
| --- | --- |
| `Received` | Adds trace headers for the relay and for the Emailit mail server that delivers the message. |
| `Message-ID` | Replaces yours with `<token@your-domain>`, where the domain is the one in your From address. Each recipient's copy gets its own Message-ID. |
| `Date` | Sets it when Emailit first processes the message for delivery. |
| `Subject` | Rewrites it from the decoded subject and re-encodes non-ASCII characters. |
| `Return-Path` | Sets a bounce address on your return-path subdomain, `emailit.<your-domain>`. The same address is used as the envelope sender, so SPF passes for your domain. |
| `X-Emailit-ID` | Adds the email's token. |
| `Feedback-ID` | Adds an identifier that mailbox providers use in complaint reports. |
| `DKIM-Signature` | Signs the message with your domain's DKIM key (selector `emailit` by default). A second signature for `emailitmail.com` can be added for complaint feedback loops. |

The DKIM signature covers the `From`, `To`, `Cc`, `Subject`, `Date`, `Message-ID`, `Reply-To`, MIME and `List-*` headers, among others. Don't change a message after Emailit has signed it, for example with a mailing list that rewrites the subject, or the signature fails.

## Headers Emailit removes

| Header | When |
| --- | --- |
| `Bcc` | Always, so Bcc recipients stay hidden. |
| `Reply-To` | When it names the same address as `From`. Some spam filters penalize a redundant Reply-To. |
| `Content-Disposition` | At the top level of the message. Attachment parts keep theirs. |

Everything else, including your own `X-` headers, is delivered unchanged.

## Control headers aren't read over SMTP

> **Use the API for per-email control:** The SMTP relay doesn't read any `X-Emailit-*` header. You can't set tracking, metadata, a template or a send time with headers. Tracking follows the sending domain's **Track loads** and **Track clicks** settings in **Email API → Domains**. For per-email control, use the [Email API](/docs/email-api/send-email/).

Don't include an `X-Emailit-ID` header in messages you submit. Emailit uses it to recognize messages it has already processed, and a message that has one skips the header rewriting and DKIM signing described above.

## The 250 reply

When the relay accepts a message, its reply to the end of `DATA` lists the Emailit email IDs it created, one per recipient:

```text
250 2.0.0 OK: queued as em_33VtK8mRq1xZp7LwN4cY2bHsDfa, em_33VtK8nB5rTq0YxW3mJk9PdLsUe
```

- `2.0.0` is the enhanced status code for success.
- Each `em_` ID is the same ID that the API, the dashboard and webhooks use for that recipient's email.
- The reply line has a length limit. If all the IDs don't fit, which happens at around 15 recipients, the reply contains only the first ID. Find the others in **Email API → Emails** or from webhook events.

Most libraries expose the reply. In Nodemailer it's `info.response`. In PHPMailer, call `$mail->getSMTPInstance()->getLastReply()` after `send()`. Save the ID next to your own record, so you can match [webhook events](/docs/webhooks/event-types/) or look the email up later with [Retrieve an email](/docs/api-reference/emails/get/).

## Add List-Unsubscribe yourself

Emailit adds `List-Unsubscribe` headers only to [campaign](/docs/campaigns/) emails. If you send newsletters or other bulk mail over SMTP, add both headers to the message yourself:

```text
List-Unsubscribe: <https://acme.com/unsubscribe?u=881&l=digest>, <mailto:unsubscribe@acme.com?subject=unsubscribe-881>
List-Unsubscribe-Post: List-Unsubscribe=One-Click
```

The `https` URL must accept a `POST` with the body `List-Unsubscribe=One-Click` and unsubscribe the person without a confirmation step (RFC 8058). Emailit includes both headers in the DKIM signature, which Gmail and Yahoo require for one-click unsubscribe.

**Node.js**

```javascript
await transporter.sendMail({
  from: 'Acme <news@acme.com>',
  to: 'ada@example.com',
  subject: 'Acme weekly digest',
  html: '<p>This week at Acme…</p>',
  headers: {
    'List-Unsubscribe': '<https://acme.com/unsubscribe?u=881&l=digest>',
    'List-Unsubscribe-Post': 'List-Unsubscribe=One-Click',
  },
});
```

**Python**

```python
msg["List-Unsubscribe"] = "<https://acme.com/unsubscribe?u=881&l=digest>"
msg["List-Unsubscribe-Post"] = "List-Unsubscribe=One-Click"
```

**PHP**

```php
$mail->addCustomHeader('List-Unsubscribe', '<https://acme.com/unsubscribe?u=881&l=digest>');
$mail->addCustomHeader('List-Unsubscribe-Post', 'List-Unsubscribe=One-Click');
```

## Related

- [SMTP settings](/docs/smtp/settings/)
- [Headers and metadata](/docs/email-api/headers-and-metadata/) for API sends
- [Email headers dictionary](/docs/dictionary/email-headers/)
- [Open and click tracking](/docs/tracking/)

---
Source: https://emailit.com/docs/smtp/headers/