Look to your agreement with the company. Usually, there is something there about what you develop or say when you are there remaining there after you leave. Unless there is some statute in your local area that protects your correspondence, you will likely have to turn everything over. If you are really concerned, consult with an employment attorney in your jurisdiction.
I am not sure if you are stating the contract has already been executed or if you are negotiating a contract with the company for 6 months. If you have already executed a contract that requires you to provide the company with your correspondence then, if the contract is enforceable, then you would be bound by the contract. If you are still negotiating with the company, then it would not be a requirement, unless you execute the contract with that clause in it.
This answer is designed for general information only. The information presented above should not be construed to be formal legal advice nor the formation of a lawyer/client relationship.