Another commenter above noted that the cache duration is on the order of "a few minutes". If the email is opened a second time later on in the day, Google's ca

Another commenter above noted that the cache duration is on the order of "a few minutes". If the email is opened a second time later on in the day, Google's cache hits the server again. (Of course, this doesn't rule out the possibility that Google is actually holding onto each version they cache. I imagine it's not actually worth it for them to do so.)