The irs keeps 6 months of emails on tape; when investigators asked for emails in mid-2013, tape was available through late 2012.
Lerner's computer crashed in mid-2011, and there is contemporaneous documentation of this crash.
2011 < 2013, it's not as if she could know that her emails would be requested in the future.
Some emails have been recovered the obvious way -- by querying the other parties.
one of many (nonmoronic) sources: http://www.motherjones.com/kevin-drum/2014/06/yet-another-ir...