What Are Email Message Headers

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∟What Are Email Message Headers

This section provides a quick introduction of email message headers.

What Are Email Message Headers? An email message header is a metadata included in the header section of an email message. Each header contains specific information that helps to send, transfer and delivery the email message.

Here are commonly used email message headers:

Bcc: Short for Blind Carbon Copy. Specifies email addresses of recipients who need to be privately informed about this email message. In other words, primary recipients and CC (Carbon Copy) recipients will not see BCC recipients.
Cc: Short for Carbon Copy. Specifies email addresses of recipients who need to be informed about this email message. Originally, the term, Carbon Copy, refers to the copy of a letter by obtained by writing on 2 sheets of paper with a carbon paper inserted between them.
Content-Disposition: Specifies "inline" to indicate that this content should be treated as an inline element. or "attachment" to be treated as a attachment file. "inline" is the default.
Content-Language: Specifies the ISO code of the language used in the email message content.
Content-Type: Specifies the MIME type code for the content of this email message. "text/plain" is the default.
Content-Transfer-Encoding: Specifies "7bit" for message content with only 7-bit bytes (ASCII characters), or "8bit" for message content with 8-bit bytes. "7bit" is the default.
Date: Specifies the date and time when the email message was initiated.
Delivered-To: Records the email address this email message was actually delivered to.
From: Specifies the email address of the sender.
In-Reply-To: Specifies the Message-ID of the original email message, if this email message is a reply.
Message-ID: Specifies a unique identifier by the first SMTP server to prevent multiple deliveries.
MIME-Version: Specified the MIME version used for the email message content.
Received: Records a log message when a SMTP server received this email message on the delivery journey.
References: Specifies Message-IDs of related email messages.
Return-Path: Specifies an email address to receive non-delivery notifications in case SMTP servers failed to deliver the email message.
Subject: Specifies a short description of this email message content.
To: Specifies email addresses of primary recipients of this email message.
User-Agent: Specifies an identifier for the MUA (Message User Agent) that composed this email message.
X-*: Specifies any non-standard headers.

See more information, see "Message Headers" defined by IANA (Internet Assigned Numbers Authority) at iana.org/assignments/message-headers/message-headers.xml