Ross,I'm sure that is correct.. but. These are not ..repeat NOT forwarded mails. At the risk of overstating this problem...
What everyone's trying to tell you that you're missing here is that you are seeing the result of spam being forwarded from user accounts. In other words, because spam has PREVIOUSLY been forwarded, hotmail has marked your server as being a spam source. Thus your new messages are getting blocked.
In other words, the problem is with the mail getting forwarded, and the only way for that to be fixed is for hotmail to learn how to distinguish between forwarded spam and a direct spam source.
Kind of a secondary result, I know.