Like the unique retinal print found in every human eye, each Internet connection has its individual IP address -- an address which makes it possible to trace email to its source. An IP address is an "Internet Protocol" address and serves to identify a specific place where information is sent from or received (in other words, a computer of some kind). Most emails contain the data needed to trace them -- although this requires a good eye and may prove to be impossible if the email sender was clever enough or used a good anonymizer.
Email headers are complicated but descriptive lists of information that give you what you need to try to trace email to its source. Although an IP address -- or, more likely, a series of IP addresses -- will be found within the header, there is no guarantee that the information you get from these IP addresses will be of any use to you. Generally speaking, email traces work better on emails that come from legitimate users because these users are not making use of various dodges. Less legitimate users will make free with viruses and Trojans to hijack other user's computers and use them to send out emails without the owner's consent or knowledge, thus giving an IP address that really doesn't tell you who sent the message, but rather who was used as a puppet to send it.
You can usually view the header of an email in your email program by clicking a special icon or an item on a dropdown menu for that email when you have it open. The header gives a list of information that ordinarily doesn't appear in your email window, but which includes an itinerary of the IP addresses the message passed through in order to reach you.
The crucial information is contained in the "Received" section or sections of the header, once you can see these by opening the header view (the method for doing this varies between email providers, so it is unfortunately not possible to describe a general technique of viewing headers here). The "Received" section records the IP address of each computer that the message passes through, beginning with the (possibly) originating computer and ending with your computer, and is one way to trace email.
An IP address is a series of numbers, such as 12.345.67.89, which identifies the computer involved in receiving or sending a message. Basically, to find the possible or probable sender of the message, you should look for the IP address which is lowest on the list. This is the earliest IP address associated with the email, and thus presumably the one belonging to the computer where the message started from.
Once you have an IP address, you can trace email to its source using an IP address tracer. There are thousands of websites on the Internet -- some paid, some free -- which allow you to trace an IP address and determine where the computer associated with it is located. Thus, tracing email is a process involving several steps, but it can be informative and useful when you're trying to determine how legitimate an email is.
Some email tracing website, such as Finding-Email.com, can provide you with the IP address and more other information after you enter an email address. You can use it as a powerful tool to trace email.