I've been looking at this today. It would seem when you send an email RIM take ownership of it and hense it doesn't originate where it appears to have (the users email address). This must make it look like SPAM. I can see why MS have implemented such a change, and it will certainly be beneficial in combatting unwated SPAM (which even a small company like ours spends thousands a year trying to cut unwanted mails out), but annoying it has a knock on effect on the BES.
As another thought I bet this has an effect on BlackBerry Internet Mail as well where users will have changed the 'Sent From' address on their BlackBerry webmail interface. I can see emails potentially getting bounced back in these instances to, although you would think RIM would have mentioned this. Perhaps I'm mistaken!