Email thumbnail URL changed to googleusercontent.com in gmail

Question

I have a system whenever user upload an image, it will send an email to the registered user\'s gmail. But in the email, i see something like this, the thumbnail is not viewable.

Answer 1

Bharata has a great and detailed answer on this, but just wanted to add one addition that I identified with a similar issue.

We had a x-webkit-csp content security header that turned out to be the culprit. Removing it and all worked through the image proxy.

Google's response was that x-webkit-csp is deprecated and to use the Content-Security-Policy header instead. However this seems like a bug that an unsupported header throws a fatal error rather than simply ignoring it.