From Neat Net Tricks:

CONFIRM.TO. There is a little-known feature that hides an HTML tag

which in turn triggers a relay system to post a read receipt to the

sender. The tag is planted in Outlook Express 4.x and Netscape Messenger

4.0 or later (and possibly any email software that supports HTML message

browsing). The message can be sent on any email software by placing

“confirm.to” in an address as: “anyuser@sample.com.confirm.to” (without

the quotes). When addressed this way, an email relay system intercepts

the mail, plants the tag in the message, and then delivers it to the

recipient to which it is addressed. When the recipient displays the

message online, the html tag triggers the relay system to send a read

receipt to the sender. No software or download is required for this to

work. The relay is performed by Postel Services. The first 30 such

relays per month are free and no sign-up is required. Greater usage is

available for less than 2 cents per receipt by setting up an account at

their site, http://www.postel.co.kr .