How can e-mail be delayed or lost?

Early hype described the Internet as an "electronic superhighway," a phrase that grew dated faster than a 1970s olive-green polyester leisure suit. In this case, however, a network of highways and side roads is a useful analogy.

If you encounter no traffic on the way to work, you can get to the office in, say, 20 minutes. But inevitably, the road is clogged with other cars, causing you to wait 5 minutes just to turn left at one intersection. Construction can make it impossible to take the usual route. Your car may break down. The trip takes a lot longer than 20 minutes.

The same things apply to e-mail traffic. Mail servers are fast, but messages can queue up under a heavy load. Internet traffic can require messages to be rerouted through paths that aren't obvious. Servers can lose connectivity with the Internet. Users can unplug network cables, "helpfully" change MUA settings (what were they thinking?), and decide blithely to send a 10MB PowerPoint file to 35 of their closest friends (and then demand to know why the message didn't arrive in nanoseconds).

It isn't common for your mail server to hand off your brilliant message to the recipient's mail server with just one "hop." Like the postal service, messages may pass from one place to another before they are delivered. Messages are handed from machine to machine in a "store and forward" model that may involve many computers, so the overall speed of delivery is highly variable.

This also means that messages travel through computers that are not visible to or even known to the sender or recipient. The store and forward model is critical to the robustness of e-mail, because it permits secondary routes for mail to get from one place to another, and for technical practices that cope with failures by taking alternate paths or retrying to create a connection when a problem is encountered.

And that's without reference to the "nasties" such as spam and viruses, the road rage of Internet traffic. In addition to consuming a vast amount of bandwidth (I could quote percentages, but any numbers I cite would be higher by the time you read this), spam, viruses and Trojan horses cause network admins to invest a lot of time and effort in building traps to prevent the bad stuff from reaching users' inboxes. Every gateway takes time, like a highway tollbooth that slows traffic.

Historically, mail servers were very forgiving of technical carelessness. But in the modern world, mail deliverability can be harmed by minor technical hygiene issues like inaccurate domain name servers (DNS), ill-considered tuning of timeout parameters and unusual mail formatting.

And then there's the burden placed on everyone for dealing with spam, phishing and viruses. There's no such thing as a perfect spam filter. They're getting better, sure, but you've probably encountered at least one situation in which a real e-mail message was stuffed wrongly into a spam folder.

Another barrier comes from misconfigured e-mail clients and servers (such as your own!) failing to follow the rules; more and more commonly, their mail is rejected by the recipient's mail server (which is not always kind enough to tell you). If that happens, the message is delayed or lost. This means that companies must enforce standards-based e-mail technology (such as ensuring that their servers adhere to the RFCs), and that users must be taught proper e-mail behavior (such as sending a message from the same server from which their e-mail ID originates).