The text at the beginning of an email message. The header 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 email header text.
The from/to, subject, cc: and bcc: fields of an email message. See cc
The actual SMTP header in an email message that is placed in front of the message.