I was looking for that nice Bounce command too, but this is what I found after googling around for a while:
The reason the "bounce" function has been removedremoved from OS X is because it does the wrong thing in the most common use case, which is to try to get spammers to remove you from their mailing list by pretending your email does not exist. The problem is that it's trivial to fake the sender of an email, so bouncing spam usually sends the bounce message to an unsuspecting additional victim; and if the same (innocent!) email address is used to fake the sender for a lot of spam, it would get inundated with bounce messages.
And this is why we can't have nice things.