I think you'd have to show they accepted the email, then chose not to deliver it to the user.
I think this is just incompetence. hopefully, it's coupled with a little incontinence.
But the thing is they're not supposed to get the email at all. The user goes into their contact book and selects the person they want to email, and suddenly Facebook is getting the email instead of it being sent to the person's actual email address.
I think Facebook's position on this would be that the user goes into their contact book, selects the @facebook.com email address of the person they want to email, and then Facebook gets the email because it's on their service.
It's not an intercept since obviously mail servers have to be able to receive email.
I fail to see what that technical detail matters. IANAL but the practical effect of this move is to intercept private communications they didn't see before without explicit permission from the user. I'd love to see a lawyer have a good go at this one.