The emails on the organisations mail server are the property of the organisation and they can send them where ever they wish.
Employees need to realise this and to only use the organisations mail system only for work related purposes.
Everyone needs to regard email messages in the same way that they regard messages sent by postcard, once out there anyone can end up reading them.
I'm also a state public servant. No such thing as an off duty PS. If you come across something unlawful you are duty bound to report it. If you don't and are found out later as having known but done nothing you are in strife.
So eg if someone sends you x email from another dept, you should send it back to that dept (someone higher up) and let them know that is happening. If it's happening within your own dept, also fwd it up for attention.
Does that help?