The e-mail spam/list creators are a different kind of adversary than, for example, web trackers.
They will do something like this: http www theirimageserver com/images/img53.jpg?to=you@email.com (Obviously they will obfuscate and use some kind of hash instead of cleartext e-mail to disguise their tracking ways).
Regardless of whether some.google.ip loads it, or your.home.ip loads it, it won't change the fact that you@email.com loaded it and your email is very active, not just active in that it didn't bounce, but active in that you actually read it.
Once again, it's a step in the right direction though, and I'm looking forward to seeing greater innovations from Google in the privacy space, because I'm confident that there are Googlers who understand that privacy is not a feature nor a PR thing... it's the difference between the preservation of humanity and society versus not.