RFC 822: Message Specification

This section explains the broader requirements of (and recommendations for) constructing a message. For example:

• Header fields are not required to occur in any particular order, except that the message body must occur after the headers.
• It is recommended that, if present, headers be sent in the order "Return- Path", "Received", "Date", "From", "Subject", "Sender", "To", "cc", and so on.
• message = fields *( CRLF *text )
• fields = dates
            source
            1*destination
            *optional-field

This section also describes the use of fields for forwarding messages, providing an audit trail to facilitate message handling (and to identify a route to the message's sender), and identifying the various hosts that sent and received the message en route. Header fields can also be used to provide information about the message's origination, in particular:

• From The identity of the person(s) who initiated sending the message
• Sender The authenticated identity of the Agent (person, system or process) that sends the message
• Reply-to Mailboxes to which responses are to be sent

Receiver fields are also described. These include:

• To  The identity of the primary recipients of the message
• Cc  The identity of the secondary (informational) recipients of the message
• Bcc  The identity of additional recipients of the message. The contents of this field are not included in copies of the message sent to the primary and secondary recipients.

Reference Fields

This section describes the fields Message-ID, In-Reply-To, References, and Keywords.

Other Fields

The Subject and Comments fields are briefly described, as are the fields used for encryption, extensions to standard fields, and user-defined fields.