Postfix + Mailman aliases problem

Hello Postfix (and hopefully Mailman) users!

Scenerio:

I have a primary mail exchanger for our domain (domainX.com) sitting in
a DMZ. All DNS MX and A records for this host (penguin.domainX.com) are
configured properly. In fact penguin is the authoritative NS for this
domain.

We had mail from the Internet comming into this server, which would run
it through Amavis/ClamAV/SpamAssassin, then re-route the mail to a
departmental MS Exchange mail server behind the firewall. Since the MX
for this domain is this very machine, I needed to forward mail to the
Exchange servers by using the Postfix transport table:

domainX.com :[192.168.1.10]

This all worked fine until we needed a mailing list solution. I
installed Mailman via RPM (I'm Fedora FC5 right now) and added a list
called "test." In mm_cfg.py I have:

DEFAULT_URL_HOST = 'lists.domainX.com'
DEFAULT_EMAIL_HOST = 'domainX.com'
add_virtualhost(DEFAULT_URL_HOST, DEFAULT_EMAIL_HOST)

When I generate a new mailing list, Mailman instructs me to add
aliases. In main.cf, my alias_maps = /etc/aliases and I do add all the
correct aliases mappings.

HERE'S WHERE THINGS GO WRONG....

If I ssh to the box and mail the mailing list from the command line
(i.e. "mail -s TestToList test") then the mail to the test list works.
However, if I make a remote connection from an MUA and send a test
message to the test list using fully-qualified syntax
(test@domainX.com) Postfix tries to deliver the e-mail to a user named
test@domainX.com at the internal Exchange server.

How do I get Postfix to analyze the aliases file and make a routing
decision BEFORE just sending anything domainX.com off to the next
relay? I suspect it may have something to do with the $mydestination
parameter, but I'm at a loss at this point. Any assistance is greatly
appreaciated!

More info:

[root@penguin postfix]# postconf -n
alias_database = hash:/etc/aliases
alias_maps = hash:/etc/aliases
body_checks = regexp:/etc/postfix/body_checks
command_directory = /usr/sbin
config_directory = /etc/postfix
daemon_directory = /usr/libexec/postfix
debug_peer_level = 2
header_checks = regexp:/etc/postfix/header_checks
html_directory = no
local_recipient_maps =
mail_owner = postfix
mailq_path = /usr/bin/mailq
manpage_directory = /usr/local/man
mydestination = $myhostname, localhost.$mydomain mail.$mydomain,
www.$mydomain, ftp.$mydomain
mydomain = domainX.com
mynetworks_style = subnet
myorigin = $myhostname
newaliases_path = /usr/bin/newaliases
queue_directory = /var/spool/postfix
readme_directory = no
relay_domains = domainX.com, domainY.com reject_unauth_destination
relocated_maps = hash:/etc/postfix/relocated
sample_directory = /etc/postfix
sendmail_path = /usr/sbin/sendmail
setgid_group = postdrop
smtpd_banner = $myhostname ESMTP $mail_name (Removed_for_Usenet)
smtpd_delay_reject = no
smtpd_sender_restrictions = hash:/etc/postfix/access
transport_maps = hash:/etc/postfix/transport
unknown_local_recipient_reject_code = 450
virtual_alias_maps = hash:/etc/postfix/virtual