What is "full headers"?

"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:

• From - The sender and the sender's email address (fakable)
• To - The receiver(s) and the receiver's email address(es) (fakable)
• Subject - The subject of the email
• Date - 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.:

• Received - Information from mail servers the message might go through
• Message-ID - Unique message identificator
• In-Reply-To - Can be found in replies, a reference (Message-ID) to the message replied
• References - 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.