To get the complete headers of an email message in Yahoo Mail Classic follow these steps:
Open the email message in Yahoo Mail Classic,
Go to the lower right hand corner of the email and click on "Complete Headers",
You should now see the complete (i.e. full) headers and not the simple headers of the message.
To return to Simple Headers, click on the "Compact Headers" link. It is located in 2 places in the email message: at the top under the complete headers on the right side and at the bottom right corner of the screen.
Yahoo! Mail (New)
To get the complete headers in Yahoo Mail follow these steps:
Open the email message in Yahoo Mail,
In the upper right hand corner of the email (above the simple headers) you'll see a link that says "Standard Header" with a down arrow next to it.
Click on the down arrow and you will see these choices: Compact, Standard, and Full
Click on Full to see the full email headers of the message (which are loaded in a pop up window)