Their email addresses are all numbers. Most email systems have Internet gateways. That means you can send mail via the Internet to people who use different email systems or services. Usually, the address within the service is translated to an Internet address. For example, if your CompuServe address was 12345,6789, your friends on CompuServe would just send their mail to 12345,6789.
Email address changes People's email addresses tend to change fairly frequently.
This can happen because of changes in a person's job, in a company's mail system, or in an individual's online service. There isn't much you can do about this except be aware of it. One of the great things about email is that you don't have to interrupt what you're doing to deal with it the way you do for a telephone call. Most of those notifiers can be turned off if they annoy you. If you don't know how to do it, check with your email system administrator.