Red Hat Linux 7 Unleashed

Red Hat Linux 7 Unleashed

By William Ball

A Brief History of Internet Email Standards

Electronic mail or email is arguably the most useful application of the Internet. (Yes, even more so than the relatively young World Wide Web.) Since the Internet's inception, there have been many public open standards published, which are called Requests for Comments (RFCs). Many of these RFCs were (and still are) related to email standards. The SMTP specification originally started with the Mail Transfer Protocol in 1980, evolved into Simple Mail Transfer Protocol (SMTP) in 1981, and since has been enhanced into the protocol we know today. (See http://www.ietf.org/rfc.html.)

Introduction to sendmail

During this time of rapid change in email protocols, one package emerged as a standard for mail transfer. sendmail, written by Eric Allman at U.C. Berkeley, was an unusual program for its time because it saw the email problem in a different light. Instead of rejecting email from different networks using so-called incorrect protocols, sendmail massaged the message and fixed it so it could be passed on to its destination. The trade-off for this level of flexibility has been complexity. Several books have been written on the subject (the authoritative texts have reached over 1,000 pages). However, for most administrators, this is overkill. sendmail was and still is written using the open source method of development. This means that all the source code is freely available and can be freely distributed. Eric has now established a company, Sendmail, Inc. (http://www.sendmail.com), that provides commercial products and add-ons for sendmail. However, the core sendmail product (now at version 8) will always remain free and open source.

One of the key features of sendmail that differentiated it from other mail transfer agents (MTAs) during the 1980s was the separation of mail routing, mail delivery, and mail readers. sendmail performed mail routing functions only, leaving delivery to local agents that the administrator could select. This also meant that users could select their preferred mail readers as long as the readers could read the format of the messages written by the delivery software.

The Post Office Protocol (POP)

With the advent of larger, heterogeneous networks, the need for mail readers that worked on network clients and connected to designated mail servers to send and receive mail gave way to the Post Office Protocol (POP). The POP RFC has undergone many revisions since its inception. The latest revision of the protocol is POP3, which has been updated a number of times since 1988. POP mail readers have since flourished: client software is available for every imaginable platform, and there is server software for not only various implementations of UNIX (including Linux), but for other operating systems as well.

POP3 does have its fair share of limitations, the main one being that it can access messages in only one mailbox (generally the user's incoming mailbox on the server). When you're reading email with a client program on Linux, such as Pine or Elm, you can create folders to manage your messages. When you're using a POP3 client, you can also create folders to organize your messages, but those folders will only exist on the machine that the POP3 client is running on. For example, if you run the Eudora email client on a Windows machine and access your mail via POP3 on a Linux server, you can save messages to Eudora folders, but these folders generally will be located on the Windows hard drive. If you use another PC to access your email with a POP3-compatible client, you will not be able to access those folders you created on the first Windows machine.

The Internet Mail Access Protocol (IMAP)

A protocol called Internet Message Access Protocol (IMAP) was developed to be an improvement on POP3. Its first RFC was based on version 4 of IMAP, so it is generally referred to as IMAP4. IMAP4 overcomes some of the limitations of POP3. The major feature is that a user can have multiple folders on the server to save their read mail. So wherever you access your email using IMAP, you have full access to all your previously read and saved messages.

Another limitation of the POP3 protocol is that it does not keep the state of messages in your mailboxes—different messages can have different states, such as read, unread, or marked for deletion. So most POP3 clients download all the messages in the user's mailbox. IMAP4 overcomes this problem by only downloading the headers of all mail items and, depending on which message is selected, downloading only that particular message. Again, there are implementations of IMAP4 servers for most major network operating systems in use today.

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