Marcus,
Your work colleague was completely out of order for referring to you in a derogatory manner in an e-mail at work. Is your company aware of this, and did you keep the e-mail. If the person attempts to press ahead with his grievance against you, I think his previous action would count against him. For the life of me, I can't see what on earth it has to do with the company, no matter what was discussed outside of work. If it was, I imagine there would be many people who would be worried about things they had said in a private conversation, heated or not.