In the context of Email Marketing, a message header is the section of the email that includes critical information about the email's sender, recipient, and other metadata. This part of the email is not typically seen by the recipient but plays a crucial role in ensuring the email is delivered and trusted by email service providers (ESPs). The message header includes elements such as the "From" address, "To" address, "Subject" line, and various technical details that aid in authentication and deliverability.