Header

This metadata covers the sender and recipient addressing, message subject, and content format.

• From: The sender name and email address displayed in the recipient's inbox and in the individual email message.
• To: The recipient name and email address displayed in the recipient's inbox and in the individual email message.
• Cc: An additional recipients' names and email addresses that received this email message.
• Delivered-To: The names and email addresses of recipients found in the To and Cc fields.
• Deliver-To: The email address that received delivery of this email message.
• Subject: The title that the sender gave to this email message.
• Reply-To: The email address that the recipient sends replies to this email message.
• Content-Type: This expresses the format of the email message's body as a Multipurpose Internet Mail Extensions (MIME) type.
• MIME-Version: The version of the MIME standard applied to this email message.
• Content-Transfer-Encoding: The mechanism used to encode this email message for transmission.

This metadata covers the route the message took across the Internet.

• Return-Path: The intended recipient email address that you sent this email message. SMTP and DNS use this metadata for email routing.
• Received: The route that the email message took. Each SMTP server adds to this header. Each instance of this pair covers one "leg" of the route, from one point to another.
• Message-ID: A unique identifier for this email message that the sending system generates.

This metadata covers the validation measures taken that confirm the providence of the email in both domain and IP address.

• DKIM-Signature: A digital signature that confirms the owner of the domain authorized this email message.
• SPF: A confirmation that the email message originated from an authorized IP address.

Of this metadata, inboxes and email messages only display To (identity of the recipient) and From (identity of the sender) in the inbox.

Some email systems add custom email headers. These have a prefix of X-. Twilio SendGrid includes an X-Message-ID header.

• Managing headers with SMTP API
• Managing headers with the Web API
• Transactional Email Templating