It is considered to be very rude to post the contents of a private email without the permission of the sender.
To the contrary, it is considered conventional wisdom never to commit anything to email that you wouldn't mind showing up on the front page of the New York Times, or writ large on the Jumbotron in Times Square. Both of those cases are just about as private as email.
If punks want to hassle M, he's more than in the right to post their missives for all to see.