E-mail
First, Alice writes a message to Bob in her e-mail program. Her e-mail program puts the message together along with some other information, such as her email address, the address of the person she is writing to, the time at which she is sending her message, and so on. When it is ready, Alice's mail program sends the message to a central computer called a mail server (or a Mail Transfer Agent) using some rules called the Simple Mail Transfer Protocol.