I gave your service a test, seeing all buttons in gray, and could not figure out if the service was broken, if my browser was broken, or if my e-mail client (Betterbird) was doing something good. Then I remembered that I use LuLu[1] to deny it all network access besides reaching my private e-mail server. Not ideal, I've learned to live with the caveats, but I do suppose it really does get the job done of stopping in-mail tracking.

[1] https://objective-see.org/products/lulu.html