Email message header provides a list of technical details about the message, such as who sent it, the software used to compose it and the email servers that it passed through on it’s way to the recipient.

One of the most useful pieces of information in the header is the IP address of the originating server, which can be used to determine why the legitimate email went to Junk E-mail folder or why your email did not go through.

To have OS X Mail display all an email message's header lines:

1. Open the message in the reading pane or its own window.
2. Select View > Message > All Headers from the menu. You can also press Command-Shift-H

To revert to the message in regular display:

Select View > Message > All Headers again from the menu or press Command-Shift-H.