[Exim] Exim blocking mail directed to mailman list(s)

Greetings:

I am in the process of performing a fresh install of Exim 4.30 on RedHat 9
to work in conjunction with Mailman 2.1.4. I have the Exim configured as
suggested in "HOWTO - Using exim and Mailman 2.1 together" on the exim.org
site. Following the configuration, I ran 'exim -bt {TESTLIST}-request' and
received:

"{TESTLIST}-request@{SERVER}
router = mailman_router, transport = mailman_transport"

But when I attempted to send email to that address, I received a failure
message:

"{TESTLIST}-request@{SERVER}: 550 Unknown user"

As far as I can tell, it's something I must not have set correctly in the
ACL, but I'm scratching my head on this one.

Any suggestions?

Brian