Understanding Mail Protocols

Internet mail delivery today is dominated by a protocol known as the Simple Mail Transfer Protocol (SMTP). This protocol is an example of a push protocol, meaning that the sending system initiates the transfer. A user writes a message using a mail reader and then tells the mail reader to send the mail. The mail reader contacts a local SMTP server, which may run on the same or another computer. The SMTP server accepts the message for delivery, looks up the recipient address, and delivers the message to the recipient system. In some cases, the recipient system may forward the mail to another system, which handles the mail account for the addressee. Depending on how the recipient reads mail, that person may use the destination mail server computer directly or run a mail client on another computer. In the latter case, the mail client uses a protocol such as the Post Office Protocol (POP) or the Internet Message Access Protocol (IMAP) to retrieve the mail from the local mail server. POP and IMAP are both examples of pull protocols, in which the recipient, rather than the sender, initiates the data transfer. This configuration is outlined in Figure 25.1. The Internet's mail system is flexible enough that the total number of links between the sender and recipient may be more or less than the number depicted in Figure 25.1, though. As already mentioned, users may read mail on the final SMTP server (inbox.pangaea.edu in Figure 25.1). The upcoming section, "Using Fetchmail," describes software you can use to further extend the chain beyond the pull mail link, as well.

Figure 25.1: E-mail typically traverses several links between sender and recipient.

Three of the computers in Figure 25.1—mail.example.com, smtp.pangaea.edu, and inbox.pangaea.edu—must run SMTP servers. These servers can be entirely different products running on different platforms. For instance, one might be sendmail on Linux, another might be Postfix on FreeBSD, and a third might be Microsoft Exchange on Windows. In addition to running an SMTP server, Figure 25.1's inbox.pangaea.edu must run a POP or IMAP server. The two end-point computers—client.example.com and franklin.pangaea.edu—need not run mail servers. Instead, client.example.com connects to the SMTP server on mail.example.com to send mail, and franklin.pangaea.edu connects to the POP or IMAP server on inbox.pangaea.edu to retrieve mail.

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