I can suggest three steps you might take to avoid the feedback: * Read messages in plain text view * Disable remote content * Read messages offline All are based on the assumption that when an email message is opened, it tries to download a hidden item (a web bug) from a server. That will tell them your current IP address, the time and the fact that you opened the message. That's just normal html type traffic. If anyone were to offer a service to check what is "leaking" then my concern would be that what you did to stop that particular service may not be effective against another. And if I were using such a CRM system myself, I think I'd try to put some essential data into graphical form, so you'd be obliged to download that graphical content to make sense of it. If you want to go about this by blacklisting, you'll be forever checking for new addresses to block. If you elect to use any of the methods above, you'll miss out on some content. I'd look at the source of some of these messages. and see if you can identify any odd-looking links. But I don't know how you could identify, generically, all such invasive links whilst still permitting those providing useful page content. Perhaps you should be looking at Tor?