The only legally reasonable expectation of an email to privacy is that it be not be intercepted on its route to the recipients from the sender. After that it is the property of the recipient. An email can be the subject of a discovery subpoena and entered into a court case as evidence.

There is no secrecy of the sender's words put in writing. Don't put in writing what you don't want disclosed. The only protection from disclosure is by separate agreement NDA, etc but that would not apply necessarily in a court proceeding.

Notwithstanding private rules of engagement; like board rules, personal ethics, etc. .