Mail responders

All these lists that have addresses to subscribe, unsubscribe, etc, how do they work? Are they using a cron job to poll the mail inbox through sendmail? Does all mail not to a specific address at the site go to a 'catchall' address, which is then polled? -- If You want to buy computer parts, see the reviews at: http://www.cnet.com/ **OR EVEN BETTER COMPILATIONS**!! http://sysopt.earthweb.com/userreviews/products/