How to track suspicious Email ?

Here one user approached me regarding an anonymous / unknown mail which he found suspicious & threat to security & he was concerned about theft of his data .
In process of investigations I had great help of one tool developed by C-DAC Thiruvananthapuram for Email forensics.

Here's the WebLink for that application :
C-DAC OnlineEmailTracer

In its simple minimalist GUI you just have to paste an Email Header of your suspicious email as an input in this tool.
& this tool will generate a route information about the email and so many other important information with which we can trace the culprits location & other vital information.

I found some other great online tools :
Here are the links :

http://whatismyipaddress.com/trace-email
http://www.ip2location.com/free/email-tracer
http://www.iptrackeronline.com/email-header-analysis.php