Email headers are the metadata fields that describe a message's origin, route and authentication.
Key points
- RFC 5322 defines header syntax and core fields: From, Sender, Reply-To, To, Cc, Subject, Date, Message-ID, In-Reply-To and References [1].
- Each server that relays a message adds a Received line, and the final server adds a Return-Path, creating a trace of the route [2].
- The Authentication-Results header, defined in RFC 8601, records the receiving server's SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail) and DMARC verdicts [3].
- Gmail requires messages to follow RFC 5322 formatting, and asks that every message include a valid Message-ID [4].
- Viewing the full header, for example with Gmail's Show original option, is the standard first step in diagnosing a deliverability problem [5].
The main header fields
A message has two parts: headers and a body, separated by an empty line. RFC 5322 defines each header as a field name, a colon and a value, and it sets out which fields are required: every message needs a Date and a From field, and should carry a Message-ID [1]. Addressing fields include To, Cc, Bcc and Reply-To. Threading fields, In-Reply-To and References, list the Message-IDs of earlier messages so clients can group replies into an Email Thread. Many other headers are added by software: MIME headers describe content types, the List-Unsubscribe Header signals opt-out options, and ESPs add their own fields such as campaign identifiers. Header names are case-insensitive, and readers usually see only a few of them.
Trace and authentication headers
Some headers exist to record what happened in transit. RFC 5321 requires each SMTP server that handles a message to prepend a Received field naming the server it came from, the server that accepted it and a timestamp, and the final delivery server records the envelope sender in a Return-Path field [2]. Reading Received lines from bottom to top reconstructs the route. The receiving server then adds Authentication-Results, defined in RFC 8601, which summarizes whether SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail) and DMARC passed and which domains were checked [3]. A DKIM-Signature header carries the cryptographic signature itself, listing which other headers it covers, so later changes to those headers break the signature.
Using headers to troubleshoot
When a message lands in spam or bounces, the headers of a delivered copy usually explain why. Gmail's help center shows how to open the full header with Show original and trace a message [5]. Check Authentication-Results first: a failed DKIM check or a DMARC failure due to missing Domain Alignment is a common cause of filtering. Then check the Received chain for unexpected relays or long delays, and confirm that From, Reply-To and Return-Path point to domains you control. Gmail's sender guidelines require RFC 5322 formatting, a valid Message-ID, and single-instance headers such as From, To, Subject and Date to appear only once [4]. For Cold Email, sending from a normal mailbox keeps headers simple and consistent.
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