postfix as virus/spam gateway only

Hello,

I'm looking to setup a postfix box to use as a mail server for some
domains (with local recipients) but also be able to serve as a scanning
gateway for other domains like so:

MX is mail.mydomain.com
postfix is spam.otherdomain.com

I want mail coming into mail.mydomain.com (say joe@mydomain.com) to be
sent to spam.otherdomain.com, scanned, and sent back to mail.mydomain.com
to be delivered.

mail.mydomain.com is running sendmail (for now) so I will figure that out
(unless one of you kind people would like to give me a hint (: ) but I
cant seem to get the postfix box to accept it. I set in the transport:

mydomain.com smtp:ip.of.mx.host

but it just rejects it saying not known in recipient table. From what I
have read this is all I have to do, but I seem to be missing something.

Any ideas?

Thanks in advance,

Bryan