It's all great, but how do they measure the open rate? From what I understand the only email client that leaks information (through css/img/font/etc back links)

It's all great, but how do they measure the open rate? From what I understand the only email client that leaks information (through css/img/font/etc back links) was Mail on iOS. Surely, you can't just extrapolate the open rate of Outlook users from that. (edit) Or do they simply send a link to webpage and then count hits?