Company emails are sent to approved people and password protected, sharing any message received or giving someone else your log in details is a disciplinary offence.
Once proprietary information is shared outside the company’s control, it’s effectively social media. Facebook, Twitter, Instagram, even PPRuNe can be accessed by anyone able to create an account. Even if you restrict your posts to friends or approved followers, there is no control over over them being passed onto third parties.
A flight attendant might take a picture of a pilot sleeping in the flight deck and post it to a company cabin crew WhatsApp group, another member thinks it’s funny and shares it on Facebook, her younger brother then shares it with his school mates, eventually it’s in a newspaper. The picture is traced back because another flight attendant informs her manager about the original WhatsApp post. The girl who took the picture gets fired.
Most companies have rules for social media and its best to follow them if you want to keep your job.