Mail relay needs to treat some addresses local

I have a Postfix mail gateway that is forwarding all e-mail for a
number of domains to an internal host. Along the way, we are scanning
for viruses with ClamAV and scanning for spam via SpamAssassin. These
two interface with Postfix via amavisd-new.

I'm trying to setup sitewide aliases that my users can use to help
train the SpamAssassin database. Now, before you say, "That's a
SpamAssassin question, go somewhere else" please read the rest of this
post.

Using this URL (http://jousset.org/pub/sa-postfix.en.html) as a guide,
I performed all the steps listed there to implement the site-wide
training aliases. The problem is that Postfix, being configured to
relay all mail, tries to relay these "bogus" domains
(whatever@spam.spam and whatever@ham.ham) to the internal host, even
though the transport file directs them to the new sa-spam and sa-ham
transports defined in master.cf.

I would guess that the fix is that some addresses need to be treated as
local by the mail gateway. Is that the fix that is needed? If so,
what is the best way to go about that?

TIA.

--
Scott Lowe