There are many parts of an email, but only three parts of an email address. Usually, when someone creates an email they try to write it so that they will remember it. This is done similar to a password. However, there are certain parts to include on an email address. People may pick their name or part of their name to use in the email address. This is the user id.
Some people identify with their hobbies or pets, so their user id names may include words associated with them. Another part of the email address is the name of the network and the final part of the email address is the domain name. Once these are all included, you can have an email address.