Content of email displays incorrectly

I have been having an unusual problem with some of my emails. Some of them seem to lose the content sent and have an old email showing instead. However, if I check my email on my iPhone, the email appears correctly with the correct content. If I move the email to a local folder on my mac, it's still wrong. If I move it back to the .mac inbox, the entire email changes so that it now shows the old email header and body.

This started about 2-3 weeks ago and it seems to pick random emails! As an example, I see that I have a new email from my cousin but when I open it up, it's an order confirmation from something a couple of years ago. I haven't changed anything on the mac that I know of.

Does anyone have any idea why this is happening?