Well, first of all the reader has to give permission to send the read reciept, but I guess it's possible with ASP to log in to a POP box where the reciepts are sent to. Parse the emails in that box and you're there. I think there are some examples on accessing a pop bix with ASP. Try www.4guysfromrolla.com or just Google.
I think you could implement some convoluted process where you send an HTML email and embed a hidden graphic with a specific, unique address.<BR><BR>Then if your server gets a request for that URL, you'll know somebody is reading it. I've never tried this and it's simply theoretical, but it's worth pondering.
Almost all e-mail clients have them. Usually in the send options or something like that.<BR><BR>It is just simply another header in the e-mail. It is the receiving e-mail client's choice to send one or not. Usually there is a setting in your client that says 1) Never send read recepts; 2) Always send read receipts; 3) Ask user.<BR><BR>ASPEmail docs have the header, I think.<BR><BR>-Doug