This How-To will explain how to get mailman mailing list working with Zimbra. I have tried this on a Fedora Core 4. You may need to adjust certain things for other OSes.

This ONLY works if Zimbra's postfix is able to deliver mail locally. One thing you should note is ALL the domains added to Zimbra are by default 'virtual_mailbox_domains', so if your hostname is set to be one of the virtual mailbox domains, Mailman will NOT work for you.
Zimbra's default install sets following in /opt/zimbra/postfix/conf/
$mydestination = localhost localhost.localdomain
You may need to change it to get the local delivery working.
I shall leave it to you to figure out how to achieve that. The variations in this regard are so many that covering all possiblities is impossible and I'm not a 'Postfix Expert' by a long shot.

List names must be unique across all virtual domains. For example, you can not have and

OK so lets get going.

First download the latest mailman sources. You can find them here.

I followed instruction on mailman documentation page.

Now, ungzip-tar, configure and install. We are going to install mailman under default location, /usr/local/mailman, user mailman and group mailman.

# As root
groupadd mailman
useradd -c''GNU Mailman'' -s /sbin/nologin -d /no/home -g mailman mailman
cd /usr/local
mkdir mailman
chgrp mailman mailman
chmod a+rx,g+ws mailman
If you want to use Zimbra's httpd running on port 7780, you need the –with-cgi-gid option. If you have another web server running, use the group id of that web server here.
# As normal user
tar xvfz mailman-2.1.7.tgz
cd mailman-2.1.7
./configure –with-cgi-gid=zimbra
Now compile and install.
# As root
make install
# following is required only if you are using Zimbra's httpd
cd /usr/local/mailman/archives
chown zimbra private
chmod o-x private
# Check your installation
/usr/local/mailman/bin/check_perms -f
Now setup the web server. Add following lines at the end of /opt/zimbra/conf/httpd.conf

#  httpd configuration settings for use with mailman.

ScriptAlias /mailman/ /usr/local/mailman/cgi-bin/
<Directory /usr/local/mailman/cgi-bin/>
    AllowOverride None
    Options ExecCGI
    Order allow,deny
    Allow from all

Alias /pipermail/ /usr/local/mailman/archives/public/
<Directory /usr/local/mailman/archives/public>
    Options Indexes MultiViews FollowSymLinks
    AllowOverride None
    Order allow,deny
    Allow from all
    # Only if you have internationalized archives
    AddDefaultCharset Off

# Uncomment the following line, replacing with your server's
# name, to redirect queries to /mailman to the listinfo page (recommended).
# RedirectMatch ^/mailman[/]*$
Now copy mailman icons and then restart apache.
# As User 'zimbra'
cd /opt/zimbra/httpd/htdocs
mkdir mmimages
cp /usr/local/mailman/icons/* mmimages/
zmapachectl stop
zmapachectl start
Now change postfix configuration.
Add/Modify /opt/zimbra/postfix/conf/ so that following varibles are set as shown.
virtual_alias_maps = ldap:/opt/zimbra/conf/,hash:/usr/local/mailman/data/virtual-mailman
alias_maps = hash:/etc/aliases,hash:/usr/local/mailman/data/aliases
Add following lines at the end of /usr/local/mailman/Mailman/ file.
MTA = 'Postfix'
POSTFIX_ALIAS_CMD = '/opt/zimbra/postfix/sbin/postalias'
POSTFIX_MAP_CMD = '/opt/zimbra/postfix/sbin/postmap'
IMAGE_LOGOS = '/mmimages/'
DEFAULT_URL_PATTERN = 'http://%s:7780/mailman/'
In the last variable, you need to list all your virtual mailbox domains for which you want to add mailing lists. Remember you should ONLY list your virtual domains here, not your local domain for which postfix delivers mail locally.

Now add your first list on virtual domain.
# As root
# first add a sitewide list called mailman, this is mandatory
cd /usr/local/mailman
bin/newlist mailman 
bin/config_list -i data/sitelist.cfg mailman

bin/newlist --urlhost=<your zimbra host> <listname> <admin-email-address> <admin-password>

# This is just to make sure that aliases and virtual-mailman files get generated. Until you add a virtual-domain based list, virtual-mailman won't be generated.

# aliases and virtual-mailman must be owned by mailman.

chown mailman:mailman /usr/local/mailman/data/aliases*
chown mailman:mailman /usr/local/mailman/data/virtual-mailman*

# setup mailman cron
cd /usr/local/mailman/cron
crontab -u mailman

# setup daemon
cp /usr/local/mailman/scripts/mailman /etc/init.d
chkconfig --add mailman
Now your should be able to visit http://<zimbra host>:7780/mailman/listinfo and browse/create new lists etc. And of course you can always use mailman's command line utilities for list maintainance.

Hope this helps.