Make sure you keep the original emails with full header...

Make sure you keep the original emails with full header information (usually hidden from view, you need to go into properties to see that)

Problem with emails as evidence is that they can be easy to fake or tamper with, and without that information they hold little weight. The only way to confirm authenticity is if they were digitally signed, or if the same item can be found in the sender's sent items (probably now deleted).

However, with the header information there is somewhat more of an audit trail. It can't prove authenticity of what was sent, but at least it can be cross-checked with server logs to show that "something" was sent.