What class/topic was the paper for? I have to write a lot of history and philosophy papers and I almost always have to argue for or against some certain idea/theory/etc. Now everyone knows that using a direct quote or verbatem language in a paper is explicit plagiarism, but a problem that usually occurs when you're writing an argumentative paper is what some of my professors have called "implicit plagiarism. That's when you develop an idea or theory within the body of your paper that you consider to be entirely your own, but has in fact been published before by someone else. The problem is, how can you even know that you're doing it? And what happens if you have an idea and you think it's your but it's coming from some other source that you may have read several years in the past?
There are a lot of professors at OU, especially in the social sciences, who feel that it's a misapplication of the conduct code to punish a student for this type of plagiarism. I had something like this happen to me in a history class where I argued for a certain point only to find out that some professor from Harvard had published a book about basically the same argument that I was presenting. My professor didn't think I had plagiarized the work because when he asked if I had ever read so-and-so's book, I was like, "Who the hell is that?" However, if I had been writing that paper for actual publication in a professional journal or a book, I could have been sued under the current intellectual properties laws. So perhaps this is the kind of plagiarism that your e-mail was about and if your professor was any kind of educator and any kind of professional he would take the time to show and explain to you what you did wrong in his estimation.