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The original 'killer app'E-mail, the overlooked killer app, is profiled, as well as qmail, Sendmail, and the latest innovations in mail-transfer agent technologySummaryBy Cameron Laird and Kathryn Soraiz |
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-mail,
of course, is that most ubiquitous application
category; its use has grown by 15 to 30 percent each year for the
past 20 years. Everyone needs and uses it. Linux users deserve to
have a particularly good relationship with e-mail, because, as we'll
explain below, Linux is particularly apt for trouble-free e-mail
service.
In this article, we'll bring you up to date on what's happening with e-mail for Linux systems, focusing on qmail and Sendmail. Plus: Look for links to an exhibitor and a tutorial at the Expo that are focusing specifically on extending e-mail's capabilities.
E-mail matters
Eric Arnum, editor of the Electronic Mail & Messaging
Systems newsletter, estimates there are 300 million in-boxes
worldwide now, a number that continues to balloon after more than
two decades of Internet growth.
E-mail is as "mission-critical" for most businesses as the other common utilities: power, water, ventilation. At its best, e-mail, like these other essentials, also works around the clock.
To the Linux community, e-mail has a special importance. Linux development depends critically on long-distance cooperation, far more often conducted via e-mail than telephone conversations, faxes, or moving molecules. More than this, though, e-mail has been a critical showcase for Linux's capabilities. It's impossible to tell how many organizations first lost their fear of Linux when an open source operating system and e-mail server took over for proprietary software that couldn't handle the load.
Even though e-mail has a long history, people are still figuring out better ways to work with it. One exhibitor booth and one tutorial at the Expo are exclusively devoted to e-mail. Before we explain what they offer, let's quickly review e-mail technology at an "executive" level.
The three heads of e-mail technology
Three broad application categories partition the mass of e-mail
applications:
- Mail-user agents, or MUAs
- Mail-transfer agents, or MTAs
- Mailing-list managers, or MLMs
An MUA is the application that resides on your desktop to receive, compose, and transmit messages. Eudora and Microsoft Outlook are examples of MUAs.
An MTA mediates everything involved in computer-to-computer transmission of e-mail messages. It receives a message from your MUA, figures out where and how to send it, and passes on the message to another MTA close to your recipient.
Mailing lists mediate much of the teamwork done through networks. MLMs simplify the chores of subscribing and unsubscribing list members, assuring that messages are securely delivered, and, in some cases, automatically preparing digests and archives of list traffic.
Many of the best e-mail programs are available at low or no charge, frequently with source code included. While MUA and mailing-list management are active areas of innovation, this article focuses on MTAs.
Any explanation of MTAs must start with Sendmail, which "is responsible for 70 percent of all e-mail delivered on the Internet," according to IBM research staffer and security expert Wietse Venema. (For a slightly lower estimate of the same quantity, see the Resources section at the end of this article.)
Sendmail, the standard for MTAs
Sendmail, first released 20 years ago, has been an open source
product ever since. Through the years, most Unix operating systems
have standardized on Sendmail as their only vendor-supported MTA.
Sendmail does it all. The February release, version 8.9.3, accumulates enhancements and corrections from hundreds of programmers who have worked through the years on performance, security, filtering, routing, portability -- anything involved in Sendmail's status as the standard for MTAs.
Sendmail is known to be difficult. Its original author, Eric Allman, says in the famous "Bat Book" (the insiders' nickname for Sendmail, Second Edition) that, "The Sendmail program is difficult to configure and even more difficult to understand completely."
The biggest event in the Sendmail world over the last year was Allman's creation of a for-profit company, Sendmail Inc., where he's now chief technical officer. (In Sendmail Inc.'s Expo exhibit, company officials will explain both their product offerings, designed to simplify Sendmail configuration, and their service contracts, created for organizations on the hunt for commercial support. At press time, Sendmail Inc. was scheduled for booth 1920 on the exhibition floor.)
Dan Bernstein's qmail
While Sendmail dominates the MTA market, it doesn't monopolize it.
Many large organizations dependent on fleets of Windows desktops
rely on commercial packages, including Microsoft Exchange, Lotus
Notes, and Novell GroupWise. Several more specialized operating
systems, including The Santa Cruz Operation (SCO), Inc.'s lines of
Unix, Compaq's OpenVMS, and mainframes generally ship with
vendor-supported packages not covered in this article.
A more direct competitor to Sendmail is qmail. As its own home page begins, "qmail is a modern replacement for Sendmail, written by Dan Bernstein." Bernstein is an acerbic, knowledgeable, and energetic researcher who designed qmail to best Sendmail's performance and security. Knowledgeable observers generally agree that he's been technically successful.
Sendmail was built for a far different world -- it almost was the case that everyone on the Internet at the beginning knew everyone else. The unimportance of security in the early years is a heritage that has yielded many problems for Sendmail more recently. Several of the most famous Internet security breaches have depended on a fault in Sendmail's security.
For Bernstein, security has been an "absolute requirement" from the beginning. He's made famous bets about the security and reliability of qmail, and lost none of them.
The market has begun to reward these virtues. Russ Nelson is the principal of Crynwr Software, a consultancy whose "main product is qmail support." He cites several high-profile, high-volume e-mail sites that rely on qmail, including the State University of New York at Buffalo, where a single SPARCstation 20 routes on the order of a million messages a day. Hotmail, the Microsoft Corporation subsidiary believed to host more e-mail addresses than any other domain, passes all outbound e-mail through qmail.
Learning qmail
Along with its performance and reliability, qmail's flexible
filtering, or aliasing, scheme attracts many system administrators.
"qmail lets you run a mailing list with zero human
intervention," notes Nelson.
That same feature is what first brought Liz Coolbaugh, director of support services for Eklektix Inc., to qmail. Coolbaugh is also managing editor of Linux Weekly News where, "we use the aliasing to separate out the huge number of mailing lists that we monitor for the LWN." You can learn more about her experience with qmail at Eklektix during her qmail tutorial at the Expo. (Coolbaugh's tutorial is Tutorial HK, "qmail/Ezmlm Installation and Configuration," scheduled for Tuesday, March 2, 1 p.m. to 4 p.m.)
Coolbaugh is teaching the session for the best possible reason: "I like qmail a great deal." She anticipates that it will be a useful tutorial for Expo attendees: "I make no claims to be a qmail expert, but I can bring other people the information they need to decide whether or not to use qmail, and make it easier for them to use it if they choose ... Because of qmail's licensing restrictions, installation is not as straightforward as just installing an RPM [Red Hat Package Manager]."
While her class will concentrate on technical points, she'll also touch on the "cultural" factors that impact a choice between Sendmail, qmail, or another MTA. As Coolbaugh says, "qmail's position in the MTA world has been negatively impacted by its license, in which Bernstein restricts the distribution of modifications to qmail and binary packages of qmail in the interest of maintaining quality, but with the disadvantage of discouraging other developers. It remains, in my opinion, the best platform for most mail-intensive sites, but is unlikely to become the default mailer as a result [of the licensing restrictions."
As is usual with licensing concerns, there's more heat than light in many discussions concerning qmail's license. It's definitely more restrictive than Sendmail's, for example. Coolbaugh reports that
On a practical, day-to-day level, the licensing is not discouraging ... The most direct way in which people are impacted comes from his restriction on the distribution of binary qmail packages. This restriction has meant that you have to untar and build qmail yourself. So the installer has had to learn more about the guts of qmail than they would have to for another product. I did not find the instructions difficult or have any problems with them, but it was certainly more time-consuming than installing the RPM-s for more liberally licensed software.
With recent relaxation of qmail's license, at least one RPM has become publicly available. See Resources for details.
Making a decision
The one other complaint we most often hear about qmail has to do
with its development staff. The development and administrative
communities have had 20 years to learn about Sendmail. Even if both
Sendmail Inc. and the nonprofit Sendmail Consortium vanished, there
would still be an abundance of Sendmail expertise in the world at
large.
qmail, on the other hand, is closely identified with a single programmer: Bernstein. It isn't that Bernstein has done anything wrong with the code he's written. qmail's implementation is coherent and consistent. The code is readable, even in its reliance on such idiosyncrasies as what Nelson calls "Bernstein chaining" -- unforked execs which frequently deliver a performance gain.
Also, users will soon have another source of comfort while working on qmail. Nelson now has a contract with O'Reilly & Associates to complement its publication of the Bat Book, with a reference on qmail.
qmail's strong design leads to several benefits besides performance, reliability, and aliasing. Nelson, for example, mentioned the ease with which he can build pager gateways, autoresponders, and sophisticated domain managers. Moreover, Nelson carefully annotates (on his qmail site) scores of user-contributed add-ons and documentation. Despite all these positives, quite a few operations with whom we've talked have decided against qmail simply on the basis that they don't want to be dependent on Bernstein.
What's important to you in picking your MTA? Ranking the following will help with the decision:
- Administrative convenience
- Security
- Performance
- Customizability
- Extensibility
- Antispam filtering
- Licensing costs
Even if you grant that it isn't maximally secure, is Sendmail "secure enough" for your situation? Does qmail's restrictive licensing matter to your organization? Which MTA will be easier to manage if you're pushing 10,000 messages a day through your Linux host? What if the level is 100,000 -- or a million? You'll probably start a comparison of Sendmail and qmail with questions like these.
Most qmail users seem to be former Sendmail users. They generally made the switch for a specific benefit. "Converts" generally cite security, the ease of mailing list management, or performance as motivations. qmail is the clear winner in each of these aspects. qmail is at least as easy to use as the no-charge versions of Sendmail, although both can be arcane.
People choose Sendmail for several reasons. It's the market leader and tens of thousands of administrators are familiar with it. Sendmail's licensing is less restrictive. Sites with specific "legacy" needs (for example, to deliver specific classes of messages through an other-than-TCP/IP telephone connection) are best off with Sendmail. Operations with special requirements are generally more likely to find someone has written an add-on or modification based on Sendmail, rather than qmail, that addresses their problems.
Perhaps the most comforting aspect of deciding between these two
options is the confidence that you can't go too far wrong. Both
Sendmail and qmail are careful to conform to published standards.
It's relatively easy to preserve all data (including old mailboxes)
while switching between one and the other. Even the worst case --
you decide on one of these two MTAs, begin using it, and then find
after several months that you've hit a security, performance, or
licensing roadblock that means your decision was the wrong one --
will cost you only a manageable total of wasted effort.
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About the author
Cameron Laird and Kathryn Soraiz manage their own software
consultancy, Network Engineered Solutions, from just outside
Houston, TX. Their Regular Expressions column appears in every
issue of SunWorld and they've written several
articles for LinuxWorld.
Reach Kathryn at
kathryn.soraiz@linuxworld.com.
Reach Cameron at
cameron.laird@linuxworld.com.