A mail server is a computer that sends, receives and stores e-mail for users. Almost every Internet Service Provider (ISP) includes at least one mailbox on their mail server as part of their basic service. Each mailbox on the server has a unique name which is usually, but not always, the logon name you use to connect to your ISP. Each mailbox also is associated with a unique e-mail address which usually consists of your logon name combined with the ISP's domain name in the form firstname.lastname@example.org
. To access mail on a server, you use a computer program called a mail client, or mail reader, such as Outlook Express.
When a client connects to a server, both computers must be speaking the same language, called a protocol. In the case of mail servers there are four protocols that can be used. The most widely used is POP3, for Post Office Protocol version 3. It is almost always used in conjunction with SMTP, or Simple Mail Transfer Protocol. POP3 is used to retrieve mail from the server (incoming) while SMTP is used to send mail through the server (outgoing). IMAP, or Internet Message Access Protocol, is a newer protocol that is not used as widely as POP3. HTTP, or HyperText Transfer Protocol, is actually the protocol used by Web servers, but it can also be used to access mail in special cases such as Hotmail. Outlook Express supports all four of these protocols. While all four protocols perform mail functions, there are important differences in how they function on the server.