Mail Server

When a mail is sent it has to reach its proper destination. In order that the mail is sent to the proper destination, the destination site has to be running a program called a mail server that listens for requests to deliver mail. The mail server does the following:

• Accept the message and store it in the expected mailbox.
• Forward the message somewhere else, usually to a place specified by the owner of the mailbox,but possibly to a mailing list.
• Reject the message as undeliverable, either because the mailbox does not exist or because the mailbox is full or because the server is facing some temporary problems.

There are basically two kinds of mail servers based on the protocol. They are:

• Basic Simple Mail Transfer Protocol (SMTP) delivery. The server translates the mailbox name into a local file name and appends the message to the file.
• Post Office Protocol (POP) delivery. The server still stores messages somewhere, in a place derived from the mailbox name. However, it allows mail-receiving connections from other Internet sites. The mail agent on the recipient's site knows to open an Internet connection to the POP server, request contents of particular messages and (optionally) remove messages from the server's mailbox.

POP service is newer than SMTP service; it has the large advantage that the mail can be accessed from anywhere on the Internet, without logging into the server.