The mail server works in conjunction with other programs to make up what is sometimes referred to as a messaging system. A messaging system includes all the applications necessary to keep e-mail moving as it should. When you send an e-mail message, your e-mail program, such as Outlook or Eudora, forwards the message to your mail server, which in turn forwards it either to another mail server or to a holding area on the same server called a message store to be forwarded later. As a rule, the system uses SMTP (Simple Mail Transfer Protocol) or ESMTP (extended SMTP) for sending e-mail, and either POP3(Post Office Protocol 3) or IMAP(Internet Message Access Protocol) for receiving e-mail.
A mail server is the computerized equivalent of your friendly neighborhood mailman. Every email that is sent passes through a series of mail servers along with its way to its intended recipient. Although it may seem like a message is sent instantly – zipping from one PC to another in the blink of an eye – the reality is that a complex series of transfers take place. Without this series of mail servers, you would only be able to send emails to people whose email address domains matched your own – i.e., you could only send messages from one example.com account to another example.com account.