# Email header

> An email header is the block of fields at the top of a message: From, To, Subject, Return-Path and more. What each field means and how to read one.

Canonical: https://www.cresca.xyz/glossary/email-header

An email header is the block of fields at the top of every email that records who sent it, who it is for, the route it took and whether it passed authentication. Mail apps show a few of these fields, such as From, To and Subject, and hide the rest. The full email header is the first place to look when an email lands in spam or does not arrive.

## The main email header fields

- From: the sender the reader sees.
- To, Cc: the visible recipients. Bcc recipients are not listed in the delivered header.
- Subject and Date.
- Reply-To: where replies go, if different from From.
- Message-ID: a unique identifier for the message.
- Return-Path: the bounce address, also called the envelope sender or MAIL FROM.
- Received: one line added by each server that handled the message, newest at the top.
- DKIM-Signature: the cryptographic signature of the sending domain.
- Authentication-Results: added by the receiving server, with the SPF, DKIM and DMARC verdicts.
- List-Unsubscribe: the address or URL behind the unsubscribe button that Gmail and other mail apps show next to the sender.

## How to see the full email header

- Gmail: open the email, click the three dots at the top right, then "Show original".
- Outlook on the web: open the email, click the three dots, then View, then "View message details".
- Apple Mail: open the email, then View, Message, "All Headers".

## How to read an email header for deliverability

Find the Authentication-Results field. It should say spf=pass, dkim=pass and dmarc=pass. A fail on any of them is the most common reason a legitimate email goes to spam.

Then compare three domains: the From domain, the Return-Path domain and the d= value in DKIM-Signature. DMARC passes when the From domain matches the domain that passed SPF or the domain that signed with DKIM. This match is called alignment.

Read the Received lines from the bottom up to follow the route. A long gap between two timestamps shows where a delay happened.

## Email header vs email body

The header carries information about the message. The body is the message itself. In the raw email they are separated by one blank line.

In Cresca

## How Cresca handles the email header

Cresca signs every email with DKIM for your verified domain and uses a custom MAIL FROM subdomain, so the Return-Path and DKIM domains align with your From address. Campaign emails carry a List-Unsubscribe header as well as the footer link. Through the API you can add your own headers with the headers field.

- [Custom sending domains](https://www.cresca.xyz/sending-domains)
- [DMARC adoption study](https://www.cresca.xyz/research/email-authentication)

FAQ

## Email header questions

### What is an email header?

It is the set of fields at the top of an email, such as From, To, Subject, Date and the authentication results, that describe the message and the route it took.

### What is the difference between From and Return-Path in the header?

From is the sender shown to the reader. Return-Path is the address that receives bounce messages. They can be different addresses, and readers do not see the Return-Path.

### Can email headers be faked?

The From field can be forged, which is how spoofing works. SPF, DKIM and DMARC exist to let the receiving server check that the email really comes from the domain in the From field.

Glossary

## Related email terms

- [Reply-to address](https://www.cresca.xyz/glossary/reply-to-address)
- [Email body](https://www.cresca.xyz/glossary/email-body)
- [Email recipient](https://www.cresca.xyz/glossary/email-recipient)

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