"however there’s not really an expectation of privacy in professional correspondence"

Uhm, yes there is. Internal company email as well as email between companies is expected to stay between the sender and receivers. Almost every corporate email footer mentions this explicitly.

I'm quite convinced that neither employers or employees would be cool with their emails being dumped on the internet for all to see. Every time that happens, it creates headlines.

Professional correspondence is very much expected to stay confined to its intended scope.