I found the solution...
I currently have Debian 5.04 +ISPConfig 3 +Postfix +Mailman working like a charm.
alias_maps = hash:/etc/aliases, hash:/var/lib/mailman/data/aliases
virtual_alias_maps = proxy:mysql:/etc/postfix/mysql-virtual_forwardings.cf, mysql:/etc/postfix/mysql-virtual_email2email.cf, hash:/var/lib/mailman/data/virtual-mailman
DEFAULT_EMAIL_HOST = 'domain.com'
DEFAULT_URL_HOST = 'www.domain.com'
POSTFIX_STYLE_VIRTUAL_DOMAINS = ['domain.com','domain2.com']
Create the list and viola all done. I am testing it in ubuntu 10.04.
I can now confirm that ispconfig 3 and mailman works 100% perfectly together with the same settings as mentioned above.
In theory this means that all Linux Os' should work with ispconfig 3 and mailman with said settings.
I have tried creating a mailman list for domain2.com...
And I kept failing, but I have finally figured it out after struggling for almost a month, and can one believe it!
The solution is so simple...
newlist -u www.domain2.com
-e domain2.com listname firstname.lastname@example.org
All done, now the admin will automatically receive an email reply stating to send mail to email@example.com
Also note that ispconfig 3 is a bit picky so you need to create a symlink for /cgi-bin/mailman in /var/www/site_domain2/cgi/
example: ln -s /usr/lib/cgi-bin/mailman/ /var/www/site_domain2.com/cgi-bin/mailman
Also remember to enable the CGI option in Ispconfig 3 web portal.