Earlier quoted context omitted.
"Email open" tracking just got a lot more reliable for all mass email & marketing automation vendors. On the flip side, those same solutions can no longer set a persistent cookie with the image, so persistent tracking based on the initial email open will stop working.
> "Email open" tracking just got a lot more reliable for all mass email & marketing automation vendors. Has it? If Google's proxy is caching images, then "email open" tracking might have broken entirely. All the sender would see is that their email has been opened once by the proxy -- for all gmail addresses put together.
Or, if they snip the GET variable for whatever reason (I don't see them doing this):
http://example.com/gmail/{yourusername}/trackingimage.php
Or even:
http://example.com/{emailidfromadatabase}/trackingimage.php
This tactic is already in use by most mass email companies.