We keep everything , forever. Storage is cheap, particularly with high compression. Lawsuits are expensive when you cannot reasonably refute allegations. Companies keep everything, it costs a couple of bucks per employee per year.
I'm not judging but this is not a good thing.
I'm involved in IT for a public company, and there is often litigation involving customers.
Your email retention policy determines how far back e-discovery can go. If your email retention policy is to keep everything forever, then e-discovery can go back as far as possible.
If your policy is to keep email archives for, say, 3 years, then e-discovery can only go back 3 years.
At least that is my understanding, and is why we do not keep everything forever.
But 6 months is absurd. And when this scandal erupted, a "legal hold" should have been placed on all of Lerner's emails, which suspends the retention policy for the litigated records.