It allows you to send and receive mail from anyone, anywhere with an email address.
Email works in a 'Store and Forward' manner.
This means that when someone sends you mail, the mail is not sent to you directly, but to your mail server. The mail will stay on the server until you ask for it. And remember the Golden Rule of Email - if you wouldn't write it on the back of a postcard, don't send it via electronic mail. This asks your mail server for any new mail since you last checked, and downloads this mail onto the machine you are running Eudora from. This allows you to send a new message.