How accurate is the location data?
Locations of your recipients are based on the IP-address of the recipient. If you have disabled IP-address tracking from your privacy settings, you won't be able to track locations. To determine what IP-address is associated to which location, we use a service that automatically gives us this information. That service gets their data directly from ISPs and their 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?.