"LISTSERV list owners' forum" <[log in to unmask]> wrote on 
06/14/2005 08:47:40 PM:

> In the local newspaper, 3 - 4 months ago, there was an item about 
someone
> offering a new service.  Subscribers would be informed 1) that the mail
> had been delivered, and 2) when the mail was opened.  And that is all
> I know/remember about that.

Spammers have been doing this for years via "web bugs", URLs in HTML 
formatted emails that are unique for each recipient.  They just parse the 
web server logs to see which URLS were requested and match it back to the 
address.

One more reason to hate HTML emails.
 
> > Not if it's coming to me, because I invariably disable the
> > function that would acknowledge receipt or opening.
> 
> And how do you do that?

Depends on the mail cleint.  In Mozilla Thunderbird on Windows, go to 
Tools - Options and it's in the advanced section.  It offers a whole range 
of how to respond depending on whether your a to:, cc: or bcc recipient.