Reference
Email headers
What the common email headers do, from From and Reply-To to DKIM-Signature and List-Unsubscribe, and which ones Emailit adds, rewrites or removes when it sends.
Updated Oct 1, 2026
Headers are the Name: value lines at the top of every email. They carry the sender, recipients, subject, routing history and authentication results. This page explains the headers you’re most likely to inspect or set, and what Emailit does with each one when it sends your mail.
Envelope and headers
An email has two sets of addresses, and they don’t have to match:
| Envelope | Headers | |
|---|---|---|
| Sender | MAIL FROM, the bounce address. Emailit sets it to your return path, an address on emailit.yourdomain.com. |
From, the address recipients see. You set it. |
| Recipients | RCPT TO, where the server actually delivers. |
To and Cc, which recipients see. Bcc is never shown. |
Receivers check SPF against the envelope sender and DKIM against the signature, then DMARC checks that at least one of those domains aligns with the From header.
How to set headers
- Email API. Use the
from,to,cc,bcc,reply_toandsubjectfields, and pass any other header in theheadersobject. See Headers and metadata. - SMTP. Emailit sends the MIME message your app submits, with the changes listed below. See SMTP headers.
Header reference
The Emailit column tells you whether Emailit adds, rewrites or removes the header, or leaves it to you.
Addresses and subject
| Header | Purpose | Emailit |
|---|---|---|
From |
The author’s address and display name, shown to recipients. | You set it. It must use a verified sending domain of the workspace, matched case-insensitively. Otherwise the API returns 422 and SMTP returns 530. |
Sender |
The address that actually sent the message when it differs from From, for example an assistant sending for a manager. |
Passed through unchanged. Emailit checks only From. |
Reply-To |
Where replies go instead of the From address. |
You set it. Emailit removes it when it contains the same addresses as From, because spam filters penalize that. |
To |
Primary recipients, as shown to everyone. | You set it. Each recipient gets a separate copy with its own email ID. |
Cc |
Visible copy recipients. | You set it. Each recipient gets a separate copy. |
Bcc |
Hidden copy recipients. | Removed from the message before delivery. Bcc recipients still get their copy. |
Subject |
The subject line. | You set it. Emailit encodes non-ASCII characters (RFC 2047) so accents and non-Latin scripts display correctly. |
Date |
When the message was written. | Set by Emailit to the time it sends the message. |
Identification and threading
| Header | Purpose | Emailit |
|---|---|---|
Message-ID |
A globally unique ID for the message, in the form <id@domain>. |
Set by Emailit to an ID on your sending domain, replacing any value you send. The API returns it as message_id. |
In-Reply-To |
The Message-ID of the message being replied to. Mail clients use it to thread conversations. |
Passed through. Set it with the headers field or in your MIME. |
References |
The chain of Message-ID values in a thread. |
Passed through. |
Routing and trace
| Header | Purpose | Emailit |
|---|---|---|
Return-Path |
The envelope sender, normally added by the receiving server at final delivery. | Set by Emailit to an address on emailit.yourdomain.com, so bounces reach Emailit and SPF is checked against your domain. You can’t change it. |
Received |
One line per server the message passed through, newest first. Useful for tracing delays. | Added by Emailit’s API or SMTP relay and by its MTA. The SMTP relay rejects a message with 550 Loop detected after more than 4 passes through it. |
MIME structure
| Header | Purpose | Emailit |
|---|---|---|
MIME-Version |
Declares the message uses MIME, always 1.0. |
Generated by the API. Passed through over SMTP. |
Content-Type |
The type of the body or part, such as text/html, multipart/alternative or multipart/mixed, and its character set. |
Generated by the API from html, text and attachments. Passed through over SMTP. |
Content-Transfer-Encoding |
How a part is encoded for transport, such as quoted-printable, base64 or 7bit. |
Generated by the API. Passed through over SMTP. |
Authentication
| Header | Purpose | Emailit |
|---|---|---|
DKIM-Signature |
A cryptographic signature over the body and selected headers, checked against the public key in DNS. | Added by Emailit with d= set to your domain and the emailit selector, plus a second signature for emailitmail.com that mailbox providers use for feedback loops. |
Authentication-Results |
The receiving server’s SPF, DKIM and DMARC verdicts. | Added by the recipient’s server, not by Emailit. Read it in the recipient’s copy to debug authentication. |
ARC-Seal, ARC-Message-Signature, ARC-Authentication-Results |
Authenticated Received Chain: forwarders and mailing lists record the authentication results they saw, so later servers can trust them. | Not added by Emailit. Added by intermediaries that forward your mail. |
Lists and automated mail
| Header | Purpose | Emailit |
|---|---|---|
List-Unsubscribe |
A mailto: or https: link that lets mailbox providers show an unsubscribe button. |
Added to every campaign email, pointing to the hosted unsubscribe page. For API or SMTP mail, add your own with the headers field or in your MIME. |
List-Unsubscribe-Post |
Set to List-Unsubscribe=One-Click to allow one-click unsubscribe (RFC 8058). Gmail and Yahoo require it for bulk mail. |
Added to every campaign email together with List-Unsubscribe. |
Feedback-ID |
An identifier mailbox providers include in complaint data so senders can group it. | Added by Emailit to every message, in the form :campaign-id:workspace-id:Emailit. |
Precedence |
An older hint such as bulk or list that some auto-responders respect. |
Not added. You can set it yourself. |
Auto-Submitted |
Marks a message as automatically generated, for example auto-generated or auto-replied, so other systems don’t reply to it. |
Not added. Set it on automated replies to prevent mail loops. |
Emailit headers
| Header | Purpose | Emailit |
|---|---|---|
X-Emailit-ID |
The email’s internal token. Emailit uses it to match bounce messages to the original email. | Added to every message. Don’t remove it if you relay Emailit mail through another system. |
X-Emailit-Tag |
A legacy header for tagging emails. | Not read by Emailit. It’s passed through like any custom header. |
X-Emailit-Meta |
The email’s metadata, base64-encoded. | Added by the API when you send meta. It stays in the delivered message, so don’t put secrets in metadata. |
X-Emailit-Tracking |
The email’s open and click tracking settings. | Added by the API when you enable tracking for a send. |