When the U.S. Forest Service was considering whether to fire whistle-blower Glen Ith back in 2007, its ethics chief was keenly aware that his emails on the topic were public records, subject to discover.  His solution?  Instruct all recipients to destroy the emails:

This information is for your eyes only.  Please destroy after reading.  It’s not protected by privilege and is subject to discovery.

Melvin Y. Shibuya
Chief, Labor/Employee Relations Branch

He repeated similar warnings in subsequent emails. 

Here is a link to an article that includes these excerpts from those emails.

Justice has prevailed, at least sort of.  Mr. Ith died of a heart attack while on administrative leave, after his job was "downsized" but his widow was recently awarded all of her litigation cost.