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IMAP serves next generation of e-mail

New messaging protocol promises advantages over POP

By David Kosiur

August  1996
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Commercial implementations of the IMAP4 messaging protocol are increasing in number. IMAP provides advantages over the time-honored POP protocol, particularly for mobile users and multipart MIME messages. IMAP offers such features as selective retrieval of messages and message parts from a server, server-based processing of messages, and shared mailboxes. (2,000 words)

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POP, the Post Office Protocol, has been the long-standing mail retrieval protocol for TCP/IP networks. But it's started showing its age at a time when users are becoming more mobile, accessing mail servers from a variety of locations, often using more than one computer. And the shift from text-only e-mail to complex messages containing graphics and multimedia attachments has further strained the capabilities of POP.

New protocols are needed. One likely candidate is IMAP, or Internet Message Access Protocol. Implementations of this protocol are finally moving out of academic environments and other test beds to broader commercial support, making it possible for corporate messaging systems to shift to IMAP.

IMAP has been designed to pick up where POP left off, offering new advantages. For instance, IMAP lets you routinely store your messages on a central server and only request the messages you want to store locally on their desktop or laptop computers. Additionally, IMAP users can perform more manipulations on their messages at the server, either by storing them in archive folders, requesting only parts of a message, or sharing a mailbox with other users.

Let's take a closer look at how the two e-mail systems work.

Models of client/server e-mail
Both POP3 and IMAP4 (the latest versions of POP and IMAP, respectively) are protocols for handling e-mail access. They each depend on another protocol, SMTP (Simple Mail Transfer Protocol), for sending mail. To see some of the differences between POP and IMAP, let's review the three basic models of client/server e-mail--offline, online, and disconnected use.

  1. The offline model is the most popular form of client/server e-mail, and is used by protocols such as POP3. In this model, a client application periodically connects to a server, downloads all pending messages to the client machine and then deletes these messages from the server. You process all of your mail locally on your client computer.

  2. The online model is most commonly associated with remote filesystem protocols, such as NFS. In this model, a client application manipulates mailbox data on a server, maintaining a connection throughout the session. The client stores no mailbox data and only retrieves data from the server as needed. You cannot do anything with your messages if you're disconnected from the server.

  3. The disconnected user model offers a hybrid of the offline and online models. In this model, a user can download some set of messages from the server, manipulate them offline, and then upload the changes at some later time. The server is again the main repository of the messages.


POP: A brief review
As mentioned, POP is the Internet's most popular e-mail retrieval protocol today. It's a relatively simple protocol to implement, and there are many client packages available for a variety of computing platforms, including Unix, DOS, Windows, and Macintosh.

POP was designed to operate mainly in the offline mode and uses the "store-and-forward" paradigm for handling your mail. You use your POP client to connect to a mail server (the "store") and download all of the pending mail to your own machine ("forwarding" the mail to your client machine). Anything you do afterwards with your mail is reflected only on your client machine, not on the server. And if you use more than one machine to check your mail with POP, you won't be able to check and cross-reference other messages you've saved on other machines.

POP can be invoked to leave messages on the server after you download them, but that depends on using a remote filesystem protocol on the server. And if you have a multipart MIME message, you're still forced to download the entire message and all its parts, unlike IMAP.

There are no provisions for sharing mailboxes or messages; if you want someone else to see a message you've received, they either have to be on the original list of recipients (as a "cc," for example) or you can forward the message to them.

The POP protocol does offer one possible advantage over IMAP when it comes to server resources. Since you're downloading messages to your client machine with POP, there's no need to consign the server's disk capacity to storing your old messages. Of course, there's the downside--you're now personally responsible for archiving your old messages.

IMAP's features
While IMAP can handle offline processing of mail, its strengths are in online and disconnected operation. In the online mode, mail is processed in an interactive fashion. You can use the client application to ask the server for only message headers, and then request only specified messages, or even parts of certain messages. For example, if you're on the road checking your mail using IMAP, you have the luxury of leaving large messages on the server, or you could decide to download a co-worker's comments about your latest presentation, but leave the edited MPEG movie that's another message part on the server until you return to the office.

Another IETF standard used in both e-mail and the World Wide Web is MIME, or Multipart Internet Mail Extensions. MIME provides a systematic way of identifying the data in the body of a message so that the mail client software can automatically decide what to do with it. In addition, MIME allows single e-mail messages to include multiple components, or body parts. Each body part can have a different type (text, image, audio, for example) and a subtype assigned to it. The subtype provides an added description of the data, to help determine what external application can handle it. (Using Sparkle to play an MPEG video file, for example).

IMAP's ability to present the user with a review of message headers and attachments before downloading from the server integrates well with MIME. You can use your IMAP client to check the sizes and types of each MIME body part before downloading, ensuring that you copy the text of a message to your laptop, but leave the attached 2-megabyte multimedia presentation on the server, for example.

Another IMAP feature is shared mailboxes. Shared mailboxes are nothing more than a server-based mailbox file with multiple-user access. The IMAP server takes care of resolving shared access to the mailboxes and messages. A workgroup could share a mailbox with minutes of their meetings, for example, or different co-workers manning a help desk could access and process messages from the same mailbox. Down the road, shared mailboxes could form the foundation for message-based groupware, particularly if you factor in server-based processing of IMAP mail and Java applets within messages.

IMAP does have a few potential disadvantages, though. Because IMAP clients offer the user more flexibility in picking and choosing which messages are to be copied to his computer, sessions can be longer than with POP as users review their mail headers and select the messages or body parts they desire. But if you're dealing with large multipart messages, IMAP's ability to leave large messages or parts on the server can save you from large download times while you're on the road.

Also, since messages are left on the server until deleted by the user, server storage may be strained by users that are reluctant to delete past messages, or simply forget to do so routinely. On the other hand, servers usually have more routine, robust backup procedures, helping guard against lost mail, should the client machine crash.

Maturing the market
Schools such as the University of Washington, Carnegie Mellon University, Stanford University, and the University of North Carolina have been using IMAP for e-mail for several years. However, only recently have commercial software developers begun to offer IMAP servers and clients. Control Data Systems, Esys Corp., ICL's Teamware Group, and Innosoft now offer commercially-available IMAP servers, with more on the way.

Between March and mid-July, SunSoft had been offering a downloadable beta of its IMAP4 message server for the Solaris operating system, called the Solstice Internet Mail Server. According to Roger Nolan, Group Marketing Manager for Enterprise Products at SunSoft, more than 4,000 copies of the beta server and more than 6,000 copies of the free IMAP client were downloaded during this trial period.

The 1.0 version of Solstice Internet Mail Server is now shipping at a single unit price of $995. The Server includes support for IMAP4, POP3, SMTP, and MIME, and ships with MS Windows DLL, which allows Microsoft Mail client software to work with an IMAP4 mail server. Roger Nolan pointed out that this product is aimed at providing corporate clients with "reliable, scalable enterprise-wide e-mail solution that utilizes open standards" and predicted that the next major version, due out later this year, would include improved server scalability (for 10,000s of users) and roll in support for Solstice Workshop for management and Java for added functionality.

To help drive the use of IMAP, SunSoft is also offering free, unsupported IMAP4 clients for Windows 3.1, Windows NT and Windows 95 as well as Solaris SPARC and Solaris x86 (versions 2.4 and greater). These clients can be used with either the Solstice Internet Mail Server or other IMAP servers. Sunsoft's planning to offer Solaris and MS Windows clients with more features and SunSoft support sometime this fall. SunSoft is also working with Esys on offering added IMAP4 clients for MS Windows and Macintosh platforms.

Other IMAP servers are on the way as well. and Netscape have both announced intentions to offer IMAP4-compatible messaging servers later this year. Netscape's announced that their Navigator 4.0 client, due out before the end of 1996, will include support for IMAP4. Version 2.0 of its Mail server will include IMAP support; a beta of the mail server is now available for evaluation at Netscape's Web site.

Since IMAP is a relatively new messaging protocol, particularly in the commercial marketplace, users may well be wary of the interoperability of IMAP servers and clients. The Internet Mail Consortium is currently planning MailConnect 1, an interoperability testing event for MIME and IMAP. It's tentatively scheduled for August 13-14 in San Jose, CA; check its Web site for more information (see the resources section below for the URL).

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About the author
Dave Kosiur, Ph.D., is an independent networking consultant and freelance writer. He has published two books on networking, including The Macworld Networking Bible ( IDG Books), which won a Computer Press Assn. award in 1995. He's now concentrating on electronic commerce, e-mail and security issues, as well as the World Wide Web (isn't everyone?). Reach David at

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