[eluser]sophistry[/eluser]
some people think this is not a Good Idea.
that said, the answer is that you insert the Disposition-Notification-To header. the email class lets you put headers into your email. only compliant (read, insecure) email clients will allow this. most people if they were aware of this "feature" in their email clients would turn it off.
there is another way using web beacons (some call them web bugs so as not to belie the invasion of privacy) embedded in email. but, that relies on images being turned on and many people these days have images turned off in their email readers.
anyhow, it's possible, but there are no airtight solutions.
