Email tracking / Read receipts with JMail

I want to be able to track when recipients have opened an email that I have sent to them using JMail. I've searched the internet for a solution and so far cannot find one. I've found these two solutions but neither will work:

• The JMail.ReturnReceipt property
  I set this to true but receive no receipt, not even a delivery receipt. When I enable logging and print the log, it shows that a receipt was sent out so I can only assume this is a delivery receipt property and not a read receipt one.
• Adding a 1x1 pixel tracking image to the email
  This would work in theory but most email clients (Outlook, Gmail, Hotmail etc) block images by default.

Any advice would be welcome. Thanks.

re: Email tracking / Read receipts with JMail

Can anyone point me in the right direction for this please?