Yahoo Mail Headers
Sometimes it is necessary for one reason or another to save the headers in an email.
The following is how to save the headers using web-based Yahoo Mail.
To save regular email headers in Yahoo Mail, copy and paste the headers into a new email or a notepad file.
Click to view larger. Paste into new email. 
You can copy it into notepad and then save the file.
To save full headers using Yahoo Mail Beta or New Yahoo Mail, open or highlight the email then click 'Actions' 
Then select View 'full header' on the dropdown menu. Press Control/A to select all of the header. Press Control/C to copy. You will see something like this screenshot. 
Open Notepad and press Control/V to paste the headers. Save the file. The headers can be difficult to read, but there are some websites to help you decipher what is there.
If you are using Yahoo Mail Classic, open the email and scroll to the bottom of the email.
Click the link, 'Full Headers'. 
Then you will see all of the headers. 
Select all of the headers, copy and paste into notepad. Save the file.