No e-mail program cam do that. They were saying that Gmail could be composed and edited off line and this was to be a Outlook killer. But Outlook has had this feature before Gmail was around.
If you go back to the article, that’s exactly what it says Gmail can do, else it would be no big deal.
I doubt very much it is possible, unless Google has come up with some other means of transmitting data that does not rely on any of the present protocols (http, ftp, etc.). Doubt that very much.
Hank