Barkley Hound
Open the specific mail and you will see "full header" under actions.
This site will take the headers and tell you the source.
http://my-addr.com/trace_email_address/free_email_trace_route/online_email_trace_route_tool.php
Arthur Dent
View the full header of an email - https://help.yahoo.com/kb/mail-for-desktop/SLN22026.html?impressions=true
Sharon
Right click on the sender name to view the headers.