"Headers" is meta information embedded in an email. If someone asks for "full headers", they refer to all information regarding the email, not only the info usual shown:
- The sender and the sender's email address (fakable)
- The receiver(s) and the receiver's email address(es) (fakable)
- The subject of the email
- Time of sending the email (fakable)
These fields do not qualify as "full headers".
There are additional, technical information hidden by the email clients, e.g.:
- Information from mail servers the message might go through
- Unique message identificator
- Can be found in replies, a reference
(Message-ID) to the message replied
- Can be found in some replies, a reference (Message-ID) to the message replied
There are several other headers. Technicians need this sort of information
in order to track the source of abuse, why a message did not go through etc.