I seem to be experiencing an issue with Mac Mail whereby every time I click on Reply, it starts a New Message rather than a Reply to the email I had selected. Also, it appears the message is then subsequently un-closable, as in nothing happens when you click on the red (x) button. The only way to rectify the issue I've found so far is to just completely Quit or even Force Quit Mail and start a new message to the person in question.