It might be AOL, or Earthlink, or MSN, or any one of hundreds of others. Individuals and companies may register their own second-level domain names if they want. This is to separate the second-level domain from the rest of the address. Nothing else will work. The rules have loosened up somewhat and now com, net, and org can be a person or any kind of enterprise.
Different countries have unique top level domains, too.
The more e-mail addresses you see, the more TLDs you'll notice. There are other variations with e-mail addresses as well. There can be more dots and additional parts than in the above example. Basically, if you have an e-mail address with parts not shown in the example up above, you're going to know about it. This is a simplified explanation.