I wasn't aware of the proxy, so I may be wrong about the GeoIP aspect, but it's possible other mail clients don't proxy, so it's something to be wary of nonethe

I wasn't aware of the proxy, so I may be wrong about the GeoIP aspect, but it's possible other mail clients don't proxy, so it's something to be wary of nonetheless. They use unique URLs for each e-mail and recipient, so when the proxy makes the request, they can trace it down to which exact e-mail was opened.