An FmH reader comments on confirm.to:

“The confirm.to trick doesn’t add a hidden tag to the email, it sends

the email to the people working at http://www.confirm.to, and then they

re-send the email, adding a receipt request to it.

This also means that when you use it, the people at confirm.to have a

copy of the email you just sent. You may not want that. Not to

mention, it only works if the recipient has HTML mail turned on.

Most modern email programs already have a confirmation option built

into it. With Outlook, for example, you can click on TOOLS, and then

REQUEST READ RECEIPT to do the same thing. Eudora, The Bat, and most

common mail programs have this as a feature.”