(1) The text at the beginning of an e-mail message. It is generated by the client mail program that first sends it and by all the mail servers en route to the destination. Each node adds more text, including from/to addresses, subject, content type, time stamp and identification data. You can trace the path of the message from source to destination by reviewing the e-mail header text.
(3) The actual SMTP header in an e-mail message that is placed in front of the message.