The header of an email is the information which precedes the body of the message  at all times. Every computer that processes the message, from the mail client which  generates it up to the computer which receives it, leaves a trace in the header.
Viewing the headers of an email is not always simply, but it is usually helpful to see the history and the origin of an email.

We urge users to send the complete header of a message to our technical staff  in the case of doubts or disputes concerning delays or incorrect delivery.

All incoming emails have their own headers, you just need to know how to "get" them from your email client.

Here below you can find out how to view the headers of emails with the most commonly used email clients. Where the process is clearly simply, you will only see  the menu/command sequence you should follow.