How can I tell if someone opens an e-mail I send? I need to know when they received or if they received it.?

before you send the email click on the "request receive confirmation" and it will let you know when they open it.

Although what the other posters are saying is true I would like to add one more thought. I have received email before while I was away from my desk...and after it sits in my preview pane for a while it is considered and marked read...even though I am not there to do so....I receive a lot of mail so if I keep getting mail...it moves further and further down the list and I may miss it unless I take the time to scroll through my list...and with so much *%#@ spam and junk mail it gets old. So...just because an email is marked read...or a program detects that has been read doesn't necessarily mean it was. Just a thought.

Unfortunately this is not a service which is available on Yahoo Mail. All you can do is try an external program such as this, follow the link. http://spypig.com/

Yahoo does not offer this with thier email service. If you use Outlook, I believe you can do it.