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.