Which component is modified as emails are transferred between mail transfer agents (MTAs)?

The email header is the component that is modified as emails are transferred between mail transfer agents (MTAs). The header contains critical information, such as the sender's and recipient's addresses, the subject line, timestamps, and routing details that can change during transit. Each MTA that handles the email may add its own information to the header, such as the "Received" fields, which log the servers the email has passed through. This modification is important for tracking the email's journey and for diagnostics related to mail delivery issues.

In contrast, the email body and attachments typically remain intact during transfer unless a modification is indicated by the use of certain protocols or if a mail server is configured to alter content due to security scanning or encoding. Email metadata, while associated with the email, usually refers to the data outlining the characteristics of the email, such as size or format, rather than being part of the content that changes during transfer. Therefore, the header's nature makes it the most dynamic component throughout the email transfer process.