This isn't Mann's critics pursuing him. This is part of a lawsuit that Mann filed against a journalist who criticized his work.
Mann filed the lawsuit, and the person he sued filed for subpoenas to get at Mann's emails because he believed that would reveal information he could use to defend the lawsuit.
This is a terrible decision, because it means you can be sued for libel (which is saying something abot someone that is alleged to be untrue) and then be prohibited from obtaining material to defend yourself (by showing that what you said is, in fact, true).
It is made worse by the fact that Mann is a government employee, because if this becomes the precedent, it will open the flood gates for government oppression via the civil court system, which has a lower standard of proof than the criminal system. If you criticize the government or its political employees, they can sue you, and you will be prohibited from obtaining evidence to defend yourself with.
"Shut up and swallow what we tell you" is basically what the court signed off on in this case.