OK. So it seems that your server is OK. Which is good!
How long has that message been returning to your queues? I must admit I'm not sure at this stage how it got into your system. That it ends up in BadMail is normal as I'll bet that hornyxgsmvycs@mail.de isn't a valid address.
At a guess, I'll say that <> sent a message to someone valid in your domain and that is how it got into your system. With a list of recipients, most mailservers would accept the message (because there was a valid recipient on the list) and relay on the message to other recipients.
I'm not sure how exchange 2000 would behave in this scenario, but this is maybe what has happened.
As Wyzard says, you won't be able to find out who <> is. Even if you could locate the message that went to one of your users, the header trail that you might obtain will likely be from servers that don't belong to the spammer.
It might be that, however it got in, it will time out within 48hrs. You can safely empty the contents of BadMail if you like.
Not much you can do.
I would still try and test your server to make sure that spammers can't relay off of it, anyway. Perhaps you can try this from a PC at home or somewhere.
Good Luck.
RobbyB