What email header field is used to indicate the primary recipient of an email message?

The email header field that indicates the primary recipient of an email message is "To." This field is specifically designed to show the address(es) of the main recipient(s) intended to read the email. When someone sends an email, the "To" field provides clarity about who the message is primarily directed towards, making it an essential component of email communication.

The "Cc" field, which stands for "carbon copy," is used for secondary recipients who should be informed about the email but are not the main targets. Similarly, the "Bcc" field, or "blind carbon copy," allows the sender to include recipients confidentially, meaning they won't be visible to others in the "To" or "Cc" fields. The "Subject" field is a summary of the email’s content, but it does not designate recipients. Thus, while all these fields play a role in email communication, only the "To" field specifically identifies the primary recipient.