Yahoo Mail

[luckyal42]

Can you add something to an e-mail which advises the sender when the recipient opens it ?

[Ethel]

Yes you can:

http://www.msgtag.com/home/

But many people, myself included, object strongly to the use of msgtag and block it, so you can't rely on it being accurate.

[stevie21]

I think Outlook has a "confirm receipt" feature but the recipient might be asked whether they want to block or enable that. You could also embed an image in the email and check at the server when the image was downloaded. Again, the recipient can disable html in emails.