Bug 921050 - Send email in the background.

Aha, so, I don't think https://bugzilla.mozilla.org/show_bug.cgi?id=921050#c26 got addressed yet, and that is a case where we would care about checkpointing our state.  Especially since this implies its own set of test coverage, I think I'm of the mind this probably wants to be a follow-up, but I figure I'd mention this here, especially as this likely does merit discussion in the comment with an XXX/TODO note or something.  (Or a comment somewhere along those lines.)