Much of the work on cybercrime unit is to discover the IP addresses among other things, to discover all kind crimes especially stalkers.
To recognize an IP address from your email I show here two examples for this job post of a case of cyber bullying.
What you should know is that you the IP addresses are important because it allows to have a geolocalization of your stalker. My intention is not show you this to open the door for you and find that persons, police will do it if you are under police support, but it's ok you can know where is the person who is bothering you, bur specially, even if you think nobody is seeing, there is a lot of information in a simple email.
In this case both messages were generated in the Mexico City and a geolocator indicates that one message was generated in a restaurant in the downtown and the other in the same area from a private network.
In case you have Yahoo Mail
Locate the action menu and click
Search the full header view action or View Full Header
You will see all the information on where the message was sent, to whom, at what time ... All this information is important to find patterns of behavior of bullies.
If your email is generated from a private company that is paying for it, then you can see the information is displayed in this way, but also has the option to complete or Full Header header:
The private couriers are much better for research purposes as if the IP address has made a loop, as when a hacker makes it is easy to determine.
But the point is that do not delete any messages from your stalker, sometimes takes time but eventually make mistakes and in the meantime, you can make a map location and even know their habits. The messages are analyzed for what they say and what they do not say.
Everything is valuable information, what people say and what they don't say.