An autoresponder is a part of most email programs, or is a script that's run on the mail server during the receipt of email.

It is, as its name implies, an email that's automatically sent to someone who just sent an email to your or sent an email to someone using a server-wide autoresponder script. "Responder" is something of a misnomer, as the email automatically sent will never have anything specifically to do with the email that was received.

99% of autoresponders are useless, annoying and bandwidth-wasting, however some have their uses. The average email recipient, say, a relatively new user of AOL or EarthLink, should not be using an autoresponder, but does so anyway. Most businesses use autoresponders to quell the worries of their customers, which is somewhat justified but sometimes just as annoying and wasteful as the average user's autoresponder.

Some examples:

Average AOLer

From: <JoeLuser31337@aol.com>

Subject: Re: ur msg

hi i got ur e-mail and i will e-mail u bak soon thanx have a great day!!!!!!!!!!

The above is a sampling of what you can expect when you get an autoresponse from someone who has no business sending an autoresponse. I would assume by now that most people know that if their message was not successfully delivered, they will eventually receive a mailer-daemon error message, and that if it is successfully delivered, no such error message will be sent, thus proving that the email was most likely delivered. An invisible form of autoresponse, if you like.

Average business

From: <support@averagedomainbroker.com>

Subject: Average Domain Broker Site Robot

Thank you for emailing Average Domain Broker. We received your message. We will respond to it in the order in which it was received.

Don't forget to check out our monthly specials! You can get deals of up to 40% off the newest, most elite domain names if you act now! Don't delay, the new, super-cool .coop TLD is available now! Get yours today! Only $50.00! Exclaimation points!

This one is more common and suffers from the same foibles as the average user's autoresponder -- it's needless, but the marketing department for (in this case) Average Domain Broker, felt that an autoresponder would be a good, if forceful way of advertising to their already-established customer base.

The problem with both of them is when they use a service that also uses an autoresponder, such as an internet service provider. Whenever either of the above has a problem with some facet of their internet connection, they will most likely email tech support. When tech support receives their email, tech support will fire away their autoresponder, thus triggering the user's autoresponder, and again triggering tech support's autoresponder, ad infinitum, until somebody sees what's going on and sets up a filter. Despite the "Presedence:" header most mail servers add to autoresponder messages, loops are still encountered, mostly because of individual people using mail programs to configure their autoresponders rather than configuring whatever mail server they're using.

I may be going out on a limb here, but I think autoresponders are just as bad as spam. Like spam, they serve no purpose other than wasting time and bandwidth, and they tell you something you already know, what with the lack of or presence of a mailer-daemon message. As more people become aware of how burdensome spam is, perhaps they will move onto autoresponders next.