Follow this configurations after installing iRedMail & Mailman.
* below method was only tested with iRedMail 0.6.1 & 0.7.0

Assumptions:
- Your server's FQDN is mail-server.mydomain.com
- iRedMail & Mailman installed on the same server

/etc/httpd/conf.d/mailman.conf

# RedirectMatch ^/mailman[/]*$ http://www.example.com/mailman/listinfo
RedirectMatch ^/mailman[/]*$ http://mail-server.mydomain.com/mailman/listinfo

/etc/mailman/mm_cfg.py

#DEFAULT_EMAIL_HOST = fqdn
DEFAULT_EMAIL_HOST = "mydomain.com"
VIRTUAL_HOSTS.clear()
MTA = 'Postfix'
POSTFIX_STYLE_VIRTUAL_DOMAINS   = ['mydomain.com']
POSTFIX_MAP_CMD = '/etc/mailman/virtual_to_transport'

/etc/mailman/virtual_to_transport

#!/bin/sh
sed -r -e 's/(^[^#][^ ]+[ ]+).+$/\1local/' $1 \
 > /etc/mailman/transport-mailman
/usr/sbin/postmap /etc/mailman/transport-mailman
/usr/sbin/postmap /etc/mailman/virtual-mailman

/etc/postfix/main.cf

alias_maps = hash:/etc/postfix/aliases, hash:/etc/mailman/aliases
transport_maps = hash:/etc/mailman/transport-mailman, proxy:ldap:/etc/postfix/ldap_transport_maps_user.cf, proxy:ldap:/etc/postfix/ldap_transport_maps_domain.cf
virtual_alias_maps = hash:/etc/mailman/virtual-mailman, proxy:ldap:/etc/postfix/ldap_virtual_alias_maps.cf, proxy:ldap:/etc/postfix/ldap_virtual_group_maps.cf, proxy:ldap:/etc/postfix/ldap_sender_login_maps.cf, proxy:ldap:/etc/postfix/ldap_catch_all_maps.cf

Set virtual_to_transport to be executable
# chmod +x /etc/mailman/virtual_to_transport

Set mailman's universal admin password
# /usr/lib/mailman/bin/mmsitepass password

Create the mailman's mailman list
# /usr/lib/mailman/bin/newlist mailman

Generate mailman's aliases
# /usr/lib/mailman/bin/genaliases

Restart services
# postalias /etc/postfix/aliases
# postalias /etc/mailman/aliases
# service postfix reload
# service mailman start

Set mailman to autostart on runlevel 3 if you're only running runlevel 3
# chkconfig mailman --level 3 on

Make sure
* the contents of /etc/resolv.conf & /var/spool/postfix/etc/resolv.conf are the same
* output of # host -t mx mydomain.com points to mail-server.mydomain.com

If you cannot send with roundcube 0.3.1, but you can send via telnet and other webmail, check your /var/www/roundcubemail/config/main.inc.php:

$rcmail_config['smtp_server'] = "%h";
$rcmail_config['smtp_user'] = "";
$rcmail_config['smtp_auth_type'] = 'LOGIN';