When viewing the email message in question, click on the "View" menu option and select "Message Source". You can also choose "View > Headers > All" from the menu and then expand the headers to show all of them. However, this view is not easily copied and if there are many headers, there might not be room to view them all. Your best bet is to view the message source directly. If the message is large, then the source will also be very large. In this case, the best bet is to choose "File > Save As > File" to save your message to a file (in EML format). You can then open this file with any text editor (like Notepad) and copy the headers …. the file is really just the message source.