A visible email header is the top section of an email that displays important information such as the sender’s name and email address, the subject line, and the date and time the email was sent. This header is visible to the recipient and allows them to quickly identify who the email is from and what it is about. It also includes options for replying, forwarding, and deleting the email. The visible email header serves as a preview of the email’s content and helps users manage their inbox efficiently.