The part of an email that precedes the body or text of the message. A message’s full header includes detailed information about the mail servers and protocols used to route the message. Most email apps, including Mail, show an abbreviated version—typically, sender, recipients, date, and subject.
In Mail, you can move the pointer around the header area of messages you receive to make a sender a VIP, and reveal icons you can use to delete, answer, or forward a message, as well as manage attachments.
When you send emails, you can add items to the header area, such as a Priority field. Click the Header Fields button in the new message window.