Staffa - 9:11 pm on Apr 29, 2010 (gmt 0)
Have you looked at the page that processes the info that the form sends to send this info in an email to you ?
It is possible that the email address that the visitor enters is used as the sender address for the email to you. Check the sender of the email you received.
If that's not the case then you will need to add the email info to the processing page.