I have to assume that email archives would be considered in the limited areas of most important recovery efforts for which files to rebuild/extract using data retrieval software and methods. AFAIK, without physical damage, these items are always recoverable, albeit at varying levels of expense. The excuse for non recovery was not expense, it was Bad Sectors. But bad sectors in this context is nonsense, it's like taking a car to a mechanic because your tranny is broken and the car repair guy saying, "sorry, tranny is broken". Fixing it is possible/highly likey and that's what these places do.
I also assume the IRS knew the importance of those emails at minimum based on Lerner's position (not necessarily related at all to targeting), but also given we have Lerner later admitting to targeting taking place as well as inquires coming in about targeting from the outside before the hard drive crash. So we now know targeting was going on during the time frame in question and we know Lerner was in some manner aware of this at this time (before/during/after crash issue).
Proving my assumptions, sure, a different ball game, one i'm not qualified for.