Robin's Mail Quest
E-mail is a wonderful invention, isn't it? Millions of people send billions of messages across the globe every single day. Ever wonder how it gets there? Sometimes e-mail arrives in seconds, and other times it can take a long time. Often it is helpful to understand how something works to figure out where the problem lies.
Let's say our hero Robin wants to send an e-mail to his sweetie Mary, on her birthday. A noble endeavor! Robin pulls up his favorite e-mail client (an e-mail client program, like Outlook, lets users read, compose, and send mail) and starts a new message. The first thing he does is type in the Mary's e-mail address, maryn@teamITS.com.
Let's break that address down a bit. The last part of the address, teamITS.com, is the domain. Similar to a ZIP code, this tells Robin's mail server where to send the message. The .com part is referred to as a top-level domain. Other top-level domains include .net and .edu, as well as country codes such as .uk and .tv. The first part of Mary's address, maryn@, is for Mary's mail server, telling it in which mailbox Robin's message should be delivered.
Meanwhile, our hero Robin composes an eloquent message congratulating Mary on surviving yet another year. Finished, he clicks his Send button, and returns to work. At this point Robin's outgoing mail server accepts the message for later delivery. In this case, Robin's outgoing mail server is handled by his company's web hosting provider, but sometimes a company will use their ISP's mail server or an internal mail server, like Microsoft Exchange or Mercury, instead. There is little difference, however sometimes a large ISP's mail system can become backlogged, delaying outgoing mail.
A short while later, Robin is startled to receive a message from a Mailer-Daemon! No, his PC doesn't need an exorcism. A daemon (generally pronounced dee-muhn) is a program that runs in the background on a server, typically a Unix server. In this case, the daemon is a mail server program.
Praying to his gods for protection, Robin examines the daemon keeping his birthday message from his sweetie, and realizes that the message is from mailer-daemon@example.com, his own mail server! It seems Robin's message has been bounced, or rejected. Oh no! Has our fair maiden rejected Robin's message? Luckily the bounce message that Robin receives will contain the reason why his message was rejected.
Our hero's heart flutters as he reads the dreaded words, User Unknown, but he quickly realizes his mistake: a mere typo has awakened the daemon! In his haste to contact the maid Mary, Robin mistyped her address martn@teamITS.com. As a result, when Robin's mail server contacted the teamITS.com mail server to deliver his message, that server blocked the transfer, telling Robin's mail server that the mailbox "martn" does not exist. Robin's mail server then helpfully returned the "undeliverable" message to Robin.
Relieved, our hero opens his Sent folder and opens the message he just sent to Mary. Since Robin uses Outlook, he can click the Actions/Resend This Message menu item to easily reopen the message. Correcting the address, he again sends the e-mail. Now, at last, his fair maiden will receive his missive and rejoice!
Once Robin received the fair maid Mary N.'s response, of course, they lived happily ever after.
September 2005
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