When choosing, check three things: are remote images off by default or loaded through a proxy, is there protection against tracked links, and can you see what was blocked? With any provider you can also simply turn off automatic image loading.
If a message says that tracking was blocked, the app stopped an image or link from reporting back. Emails tracked by HawkOpen also carry their own notice and the link “Do not track my activity”, where you can refuse tracking by the sender’s company.
Where rules that require consent apply, tracking may start only after that consent has been obtained. The link in the message lets the recipient withdraw consent later.