Doesn't gmail already solve this problem? The way gmail solves it is by downloading all images in the email, even before you open the email. That means marketers don't know if you really opened the email. Gmail also downloads the images through proxy servers, so marketers won't know your ip address.
The DuckDuckGo service isn't claiming to be a proxy for all of your online accounts, so of course logging into your email provider to check your email means they get your IP address, user agent, etc. How else would you expect this to work?
https://gmail.googleblog.com/2013/12/images-now-showing.html