Header
The structured metadata block at the beginning of an email message, containing fields like From, To, Subject, Date, and numerous technical fields that describe how the message was composed, routed, and encoded.
Every email message consists of two parts separated by a blank line: the header section and the body. The header is a series of name-value pairs, one per line, each following the format "Field-Name: value". RFC 5322 defines the syntax and the meaning of common fields. Headers accumulate as a message travels: each mail server adds a Received header, creating a traceable delivery chain.
User-facing fields like From, To, Cc, Subject, and Date are the most recognizable headers, but there are dozens of technical fields as well: Content-Type describes the body encoding, MIME-Version signals multi-part structure, Message-ID provides a globally unique identifier, and X- prefixed fields carry vendor-specific metadata (such as Gmail's X-Gmail-Labels).
Headers are the primary data source for sorting, filtering, and searching email. Mbox Viewer parses headers for every message in an MBOX file to populate the column list — sender, recipient, subject, date — and to build the full-text index. Searching with filters like from:alice@example.com or subject:invoice targets header fields directly.
Related terms
Multipurpose Internet Mail Extensions — the standard that defines how email messages encode non-ASCII text, HTML bodies, attachments, and other binary content within the plain-text structure of email.
A globally unique identifier assigned to each email message, specified in the Message-ID header. It is used to track messages, build conversation threads, and detect duplicates when merging archives.
The delivery metadata used by SMTP servers to route an email message — specifically the envelope sender (MAIL FROM) and envelope recipients (RCPT TO) — which may differ from the visible From and To headers.