Inside Higher Ed has an article dealing with a request for emails from a public university. While the story is somewhat different from past tales - usually, it has been a right wing group going after a left wing professor and this is the opposite - the basic lesson is that it is highly likely that if someone requests your emails as public records - he/she will get them. Or at least you should assume so. Even if you delete your past messages, they live on in a recipients computer which might also be at a public university.
There are issues of privacy, etc. One can argue that in a Good World, you would be able to keep your emails private. But that is not where we live. Use the phone for private communications. And we know even that may not do it! Perhaps, whispering?: