If you add a Disposition-Notification-To header, filled in with your address (with the same address format as your From header), then many email clients MAY send a read receipt when the message is read. From the email you get back, read the headers to find out what machines it has went thru to get to you. That having been said, many email clients either ignore or do not implement returning read receipts, or ask for confirmation before sending, so the reader may not permit the receipt.

Beyond this, I would suggest a look at the appropriate RFCs. While there are such things as web bugs and such, such things tend to draw large amounts of ire from those who vigorously defend their privacy, and, because such devices are commonly used by spammers, may cause your messages to be classified as spam, possibly preventing your message from reaching its recipient(s).

I suspect, however, that if you reexamine your application you may find that you do not need to know the actual machine the message was read from, just that the message was received and opened (but this is a guess).