As concerns forwarding another person’s email to a third party without the sender’s consent, any potential legal liability is likely to be civil, rather than criminal, and would be highly fact-specific.
Possible sources of liability might include (but would not necessarily be limited to):
Breach of confidentiality obligations
Intentional infliction of emotional distress
Copyright infringement, though the “fair use” defense easily could be available
Bottom line: There is no yes-or-no answer to this question that will apply in all situations.