You probably have seen the location panel on your mail tracking page. And now you are wondering how the location of the recipients is determined or how accurate it is.
The location data is based on the IP-address of the person that has received the tracking pixel. For this we use a service that automatically gives us the location data. That service gets their data directly from ISPs and that data gets updated regularly. This way we can provide a fairly accurate visualization.
However, there are some exceptions. If the recipients of the tracking pixel uses a VPN or their e-mail service uses a proxy, then location data of that recipient won't be tracked. But rather the location data of the IP-address of that proxy. This frequently happens when sending tracking pixels to Gmail users, then the IP-address of the GoogleImageProxy will be tracked. You can read more about this in What is GoogleImageProxy?.