So we need a cloud service for "read receipts"? Interesting.
Such pixel are only used by newsletter or shaddy spam senders.
Good that Outlook and other mail apps blocks third party pictures by default.
I can think of one email client with ~15 million users that does that.
https://support.google.com/mail/answer/145919?co=GENIE.Platf...
K9, gui Android (think it blocks by default, or I turned off auto load when I installed ages ago).
And of course, a lot of email clients only open such images on demand anyway.
http://blog.mailchimp.com/how-gmails-image-caching-affects-o...
Since Google's goal with this was clearly to defeat tracking, I would strongly expect the former, but I can't back this up.