What are Email Headers?

Email headers are code snippets in any email containing various information about the sender, recipient, email MTA route or details. All email headers always precede the email body.

Why would you care about email headers?

Email headers are an important part of your email marketing, even though you as a marketer do not interact directly with them.

They have an important role in deliverability, email authentication and in the technical communication between systems.

Email headers are represented by key value pairs, where key represents the name of the header and value… well, the value for that name.

Roles & benefits of headers added to outgoing emails

  1. provide information about the sender and the recipient
  2. prevents spam by authenticating the sender of the email
  3. help identify the email route by telling MTAs (Mail Transfer Agents) where to direct the email to. Each MTA adds a bit more information to the message and the time it was received and forwarded.

Most important email headers

While there are quite many possible email headers that systems will add automatically (or you can do it yourself, maybe without even knowing it), the most important ones are the following:

  • To
  • From
  • Body
  • Subject
  • Reply-To
  • Content-Type
  • CC and Bcc (Carbon Copy and Blind Carbon Copy)
  • DKIM-Signature
  • Received-SPF
  • Priority
  • Encoding
  • Encrypted

How to view email headers?

Head over to your email client. Let’s say this is Gmail.

Click the 3 dots on the right side of every message and select Show original.

View original message in Gmail

A text list like the following will show up on the screen.

How email headers look like

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Full List of email headers

Header NameProtocolStatus
Accept-Languagemail
Alternate-Recipientmail
Archived-Atmailstandard
Authentication-Resultsmailstandard
Auto-Submittedmailstandard
Autoforwardedmail
Autosubmittedmail
Bccmailstandard
Bodynonereserved
Ccmailstandard
Commentsmailstandard
Content-AlternativeMIME
Content-DescriptionMIME
Content-DispositionMIME
Content-DurationMIME
Content-featuresMIME
Content-IDMIME
Content-Identifiermail
Content-LanguageMIME
Content-LocationMIME
Content-MD5MIME
Content-Returnmail
Content-Transfer-EncodingMIME
Content-Translation-TypeMIMEstandard
Content-TypeMIME
Conversionmail
Conversion-With-Lossmail
DL-Expansion-Historymail
Datemailstandard
Deferred-Deliverymail
Delivery-Datemail
Discarded-X400-IPMS-Extensionsmail
Discarded-X400-MTS-Extensionsmail
Disclose-Recipientsmail
Disposition-Notification-Optionsmail
Disposition-Notification-Tomail
DKIM-Signaturemailstandard
Encodingmail
Encryptedmail
Expiresmail
Expiry-Datemail
Frommailstandard
Generate-Delivery-Reportmail
Importancemail
In-Reply-Tomailstandard
Incomplete-Copymail
Keywordsmailstandard
Languagemail
Latest-Delivery-Timemail
List-Archivemail
List-Helpmail
List-IDmail
List-Ownermail
List-Postmail
List-Subscribemail
List-Unsubscribemail
List-Unsubscribe-Postmailstandard
Message-Contextmail
Message-IDmailstandard
Message-Typemail
MMHS-Exempted-Addressmail
MMHS-Extended-Authorisation-Infomail
MMHS-Subject-Indicator-Codesmail
MMHS-Handling-Instructionsmail
MMHS-Message-Instructionsmail
MMHS-Codress-Message-Indicatormail
MMHS-Originator-Referencemail
MMHS-Primary-Precedencemail
MMHS-Copy-Precedencemail
MMHS-Message-Typemail
MMHS-Other-Recipients-Indicator-Tomail
MMHS-Other-Recipients-Indicator-CCmail
MMHS-Acp127-Message-Identifiermail
MMHS-Originator-PLADmail
MT-Prioritymailstandard
Obsoletesmail
Organizationmailinformational
Original-Encoded-Information-Typesmail
Original-Frommailstandard
Original-Message-IDmail
Original-Recipientmailstandard
Originator-Return-Addressmail
Original-Subjectmailstandard
PICS-Labelmail
Prevent-NonDelivery-Reportmail
Prioritymail
Receivedmailstandard
Received-SPFmailstandard
Referencesmailstandard
Reply-Bymail
Reply-Tomailstandard
Require-Recipient-Valid-Sincemailstandard
Resent-Bccmailstandard
Resent-Ccmailstandard
Resent-Datemailstandard
Resent-Frommailstandard
Resent-Message-IDmailstandard
Resent-Sendermailstandard
Resent-Tomailstandard
Return-Pathmailstandard
Sendermailstandard
Sensitivitymail
Solicitationmail
Subjectmailstandard
Supersedesmail
TLS-Report-Domainmailstandard
TLS-Report-Submittermailstandard
TLS-Requiredmailstandard
Tomailstandard
VBR-Infomailstandard
X400-Content-Identifiermail
X400-Content-Returnmail
X400-Content-Typemail
X400-MTS-Identifiermail
X400-Originatormail
X400-Receivedmail
X400-Recipientsmail
X400-Tracemail
IANA email headers list

If you want to analyze email headers you can use this tool from Google. It can help you:

  • detect delivery delays
  • aproximative source of the delay
  • responsible of the delay.

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