Email isn't really a publishing tool however. It's apples and oranges. If the pedos on Tumblr were DMing each other with the pics (Tumbler has DMs right?) then Apple wouldn't have known about it and shut them down.

That said, if they aren't actually hosting any of the content they probably won't run into the same problem Tumblr did. They will be able to pin the blame on third party servers. The app store still has a Tor client for example, because the Tor project doesn't host any user editable content. It's when someone loads a CP image from movim.eu that they get in trouble.