Header field
One entry in an email header: a field name, a colon and a value, such as Subject: Your invoice, possibly folded across several lines.
What it means
RFC 5322 defines a field name as printable ASCII characters other than the colon, followed by a colon and the field body. Names are matched without regard to case. Some fields may appear at most once (From, Sender, Reply-To, To, Cc, Bcc, Date, Subject, Message-ID, In-Reply-To, References), others may repeat (Received, Comments, Keywords), and any field the standard does not define is allowed as an optional field and ignored by software that does not know it.
IANA keeps a registry of header field names and the documents that define them. Repeating a single-instance field is a frequent cause of rejection: Gmail refuses messages with duplicate headers, and Google's guidelines name From, To, Subject and Date as fields that must appear only once.
Example
Subject: Your invoice for OctoberRelated terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 5322, section 2.2: Header Fieldsrfc-editor.org
- RFC 5322, section 3.6: Field Definitionsrfc-editor.org
- IANA, Message Headers registryiana.org
- Google, Gmail Help: Email sender guidelinessupport.google.com
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