Somebody backs up their computers. Perhaps it's done internally, perhaps some other contractor does it... but someone does it. No company operates (especially for senior people like Lerner) without doing backups.
The IRS is stonewalling. The only way to get the answers will be if some brave (and suicidal) soul in the organization decides to be a whistleblower.
All they’d have to do is forward the emails to Issa, Gowdy, etc. from a library computer.