| Introduction to E-Mail Technology |
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How does e-mail get from the sender to the receiver?A. Perhaps the first fundamental is that e-mail isn't handled by one kind of server or technology. It's a suite of protocols that are served by distinct processes. We'll look at those in a little more detail after the overview. Let's say you've written a brilliant message in your e-mail client—the software application you use on your desktop to compose and organize messages, such as Microsoft Outlook, Apple Mail or Thunderbird. E-mail professionals call that client application the mail user agent (MUA). The MUA may not be a desktop application; it may be a "Web mail" application that runs on a Web server and which you control using your browser. Web mail clients, whether through Gmail, Yahoo or a corporate front end to another system (say, to Lotus Notes), are treated the same way as desktop client MUAs by the rest of the e-mail transport process. When you click on the Send button, the message disappears from your screen... and sets an entire chain of events in motion. After you click Send, the message is transferred to your outgoing mail server, which is probably named something like mail.yourcompany.com. The mail server—formally called a mail transfer agent (MTA)—knows to accept the message, either because you are in a network it trusts, or because you provided a username and password (generally stored in the MUA's configuration files). This network process is accomplished using the Simple Mail Transfer Protocol (SMTP), and the "make sure the sender is trustworthy" process is called authenticated SMTP. With your brilliant message in hand (or in queue), your mail server needs to send it along. The mail server contacts the recipient's mail server and transfers the mail, again using SMTP. But of the millions of mail servers, which does it contact? Your mail server does a lookup on the domain name servers (DNS), which are a kind of library card catalog for the Internet, to find out who's signed up to accept mail for the recipient's domain. The DNS gives your mail server the mail exchange (MX) records (there can be more than one) that are registered for that domain. That gives your mail server the server to contact, and it can start on its "Hey, I've got mail for you" conversation. The message is sent over the Internet, via TCP/IP (Transmission Control Protocol/Internet Protocol). Don't generalize and say "over the Web," here; while you can occasionally use the terms interchangeably, this isn't one of those times. Hearing you say this will make your techies wince. The server-to-server communication process is somewhat different than it is when the server is talking to the client MUA, although both use SMTP. One difference is trust; between hard-coded programming and administrator settings, every mail server through which a message passes—and there may be several—has to assume that the message is wrongly formatted (like the post office refusing a letter because it lacks a full street address) or, sadly more likely, because it breaks the rules in the pursuit of sending spam or viruses. Primarily because of spam, most mail servers put each message through a multistep process before they will even accept the data, much less store it and forward to the user. Those steps are covered a little more below, and in some detail in Getting Clueful: Five Things You Should Know About Fighting Spam; for this broad overview, just be aware that messages can be lost or rejected for many reasons, not all of which are intended to cause you personal grief.
Once the message arrives at the destination mail server, that is, the server responsible for delivering to the recipient (such as mail.yourcustomer.com), it's ready to distribute to the individual who is, presumably, anxiously waiting for a word from you. Here, too, there are choices for the mail administrator, particularly in how mail should be stored and forwarded to end users. Every organization (or its e-mail admins) decides which method best serves its needs. Most likely, the primary protocol used in your shop is the Internet Message Access Protocol (IMAP), which keeps all messages on the incoming mail server, neatly sorted into user folders. It's far more rare, nowadays, for companies to use the Post Office Protocol (POP3). Using POP3 e-mail, the "Get new mail" command in your MUA causes the application to download all messages to the local computer. Under most circumstances, the POP3 e-mail messages are then deleted on the mail server. The recipient presses "Get new mail" on her own MUA... and there is that brilliant message you wrote. Magic! By the end of the process, your e-mail message may travel around the world through five or six separate computers. But in many cases, your brilliant message arrives on the recipient's desktop in a minute or two. Is that cool, or what? All that happens quickly when the system works. But what happens when it doesn't? |