Guys : - What you need is print out of email along with header " Header is generally not visible to you" But you need to change littel setteings to make it visible.
Header of email will contain source and destination IP Address ......
Now if opposite party challange it then you can ask court for FSL (Forensic testing of email) or you directly contact Forensic lab with CD containing header ..They will mention 2 things.
1. It is not spam email , means email address from it was sent it valid.
2. IP address of source and from where email was generated.
3. And approximate location of IP address , Like they will mention that its generated from 192.168.0.1 IP which belong to BSNL Broadband Connection Airtel (example only)
Now You can call Yahoo Gmail to Court and ask to whom whis email ID belong and when it was created and other details of email address