[exim] Something I wish SMTP had ....

One thing that would be nice in the SMTP protocol is a redirect response
so that when a message came in you could tell the sending server to go
to a different host to deliver the email. That way you could dynamically
point mail to spam filters which would then clean the email and send it
back. Wonder what it would take to get the SMTP protocol changed? It
could be an extremely useful feature.