If someone you don't know sends you an email, it is natural to want to know who the person is, even if the email is not spam or unfriendly. Knowing a sender's identity can tell you whether or not you want to reply to the email. Some emails contain the name of the person who sent it, and even if it doesn't, some email programs will tell you the name the email account is registered under and include the information in the same line as the email address. If this information is not available, sometimes you can learn more by looking at the email address itself.
While an email address like catlady@emailprovider.com may not give you a name, the domain name (the word between the @ and the .com part) can give you a clue. Major email providers like yahoo and lycos won't tell you much, but the domain is something unusual like 'rockhounds.com' or 'businessname.com', you can perform a simple Google search for the domain name. This can narrow down the possibilities quite a bit. If someone you don't know is sending you email from a business email address, you can contact the company for more information. It could be that the person had a legitimate reason for contacting you but is unaware of proper email etiquette.
Spam email, on the other hand, can be nearly impossible to trace for the average citizen. Most people choose to simply delete these emails or mark them as spam so that they don't have to deal with them anymore. On the other hand, mysterious or harassing emails warrant more attention and can be quite a headache to track down.
It can be difficult if not impossible to find the owner of an email address if the person has used a false name to register the email account. If someone has chosen to use a bogus name and address to register for an email account with a major provider, you have little power to find the information as a private citizen. In these cases, where someone is harassing you and you need it to stop, it is advisable to enlist the help of a reputable company to find out who is doing the emailing.
There are a few companies on the internet that will allow you to do a reverse email search to find out who the account is registered to. These companies generally charge a small fee per search, or you can register for a membership and conduct as many reverse email searches as you like. However, if you are able to find out the true identity of a harassing emailer, it may not be a good idea to try to handle the problem yourself.
If you can find out who owns an email address through one of these services, you next step should be to report the person to the proper authorities. Be sure to save any harassing emails to use as evidence. Companies that conduct these searches can sometimes help you with the legal process required to put a legal stop to unwanted internet behavior.