[skloubkov]

Seems like a privacy hole.

Mailer can setup url that is composed of random words and is unique per email.

Ex: www.tracker.com/weather-dog-city-nice.jpg could identify you + timestamp of request and bam, you have record of: valid email, address isn't blocked and that user reads emails from recipient. No proxy in a world would be able to make this request anonymous.

No idea what advantage of this is apart from google eventually offering an alternative to gmail (think of comment system being replaced on youtube)