Me, I would not like the redirector to block spam: it is up to me to decide what is spam. What if it bounces something that I want? As it is not an actual mail service, there is no way in which I can tune the filters for /my/ account.
I'm pretty sure that you would not want anything that came from a SMTP client that is on a blacklisted IP address, or not properlky re-sending its mail when the SMTP server tells it to "come back in five minutes".
Greylisting and dns based block lists should at least be an option.
cheers
MH