In an email message header, where must one add a recipient's email address?

In an email message header, the recipient's email address must be added in the "To" field. This field is specifically designated for indicating the primary recipient of the email, making it clear who the main intended audience of the message is.

The "To" field is critical for effective email communication as it directs the email to the intended recipient, allowing for proper delivery and context. This ensures that the email is routed correctly and that the recipient understands they are being directly addressed.

The other fields serve different purposes: "CC" (carbon copy) is for recipients who should receive a copy of the email but are not the primary audience, creating transparency among multiple recipients. "BCC" (blind carbon copy) also allows for distribution to multiple recipients without disclosing their addresses to each other. The "From" field indicates who is sending the email. By using each field appropriately, users can maintain clarity in their email correspondence.