I found a security hole in Panther's Mail.app today. :-(

I isolated the problem and have submitted a report to Apple's bug reporter. I won't say anything more about the bug here, except that I sure hope it's fixed quickly! It could end up being pretty embarassing for Apple and Mac advocates if it's not fixed quickly, as it's the kind of thing you would have expected from Outlook Express, not Mail.app.

In the mean time, I don't think it's so serious that you shouldn't use Mail.app, because I don't think it could be used to transmit or damage any information on your computer, for example, but perhaps I'm not being creative enough in thinking how it could be abused.