> I don't understand how a proxy will protect me from an image loaded as

It will protect you from it being as a read receipt if (and I'm not sure if this is the case, though it should be trivial to test by sending email with images served from a site you control to an email you control without opening it) Google requests the image once it has received the email.

If you read other comments, a number of people have done the trivial test and confirmed that images aren't requested until you open the email.

Wow this is really dreadful.

Now it will seem to the sender that 100% of e-mails sent to Gmail recipients have been opened, rendering actual measurement impossible. A unique ID is useless if all unique IDs are requested all the time.

I'm sure they built-in rate-limiting to prevent DDOSing the sender's image server...

Apparently not. First, only messages read in gmail.com and Gmail mobile clients (not any other email clients). And apparently the image is not requested from the sender until the message is opened. Finally, apparently any querystring parameters will continue to function as usual.

Or possibly they could use heuristics to detect a newsletter and deliver the first fetched image to every recipient of the same newsletter.

Even if Gmail loads tells the marketer that you have opened the message (as people are pointing out, it's a bit unclear), there are still some advantages.

It protects you from the guys at marketer tracking your user-agent, your ip address (which gives a rough geo-ip), the number of times you opened the email, etc. It's unclear to me whether the images could set cookies before (they probably did), but even without that, they could just use etag-tricks, or stuff like that, to track you cross-sites.

Marketers might now know when you open the message, but proxying the image prevents them from getting more precise information.

(No idea exactly how much of this and more Google does, obviously, but they put themselves in position to do it)

It will if the images are cached.

No it wouldn't. You can't tell anything about what's on the other end of random_unique_number_65984654.jpg