Not complete!!'''
Here is the Fullversion ->http://www.isp-control.net/forum/trac-spam-t-342-3.html#pid5950

If you want to have a related URL to Customers webmail like:

webmail.customer-domain.tld

you must edit your 00_master.

1.) Open the 00_master

# vi /etc/apache2/sites-enabled/00_master.conf

2.) Jump to the End and add these lines.
Change "YOUR_IP" and "YOUR_ADMIN_EMAIL".

# related URL to customers webmail
<VirtualHost <YOUR_IP>:80>

    ServerAdmin     <YOUR_ADMIN_EMAIL>
    DocumentRoot    /var/www/ispcp/gui/tools/webmail

    ServerName      webmail.admin.isp-domain.tld
    ServerAlias     webmail.*

    ErrorLog        /var/log/apache2/users/webmail-error.log
    TransferLog     /var/log/apache2/users/webmail-access.log

    CustomLog       /var/log/apache2/webmail-traf.log traff
    CustomLog       /var/log/apache2/webmail-combined.log combined

    <IfModule mod_fastcgi.c>
           SuexecUserGroup vu2000 vu2000
    </IfModule>
    
    <Directory /var/www/ispcp/gui/tools/webmail>
        Options -Indexes Includes FollowSymLinks MultiViews
        AllowOverride None
        Order allow,deny
        Allow from all
    </Directory>

   <IfModule mod_fastcgi.c>
        ScriptAlias /php4/ /var/www/fcgi/master/
        <Directory "/var/www/fcgi/master">
            AllowOverride None
            Options +ExecCGI MultiViews -Indexes
            Order allow,deny
            Allow from all
        </Directory>
    </IfModule>

    <IfModule mod_php4.c>
        <Directory /var/www/ispcp/gui/tools/webmail>
            php_admin_value open_basedir "/var/www/ispcp/gui/:/etc/ispcp/:/proc/:/var/www/virtual/:/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin"
            php_admin_value session.save_path "/var/www/ispcp/gui/phptmp/"
        </Directory>
    </IfModule>

    <IfModule mod_php5.c>
        <Directory /var/www/ispcp/gui/tools/webmail>
            php_admin_value open_basedir "/var/www/ispcp/gui/:/etc/ispcp/:/proc/:/var/www/virtual/:/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin"
            php_admin_value session.save_path "/var/www/ispcp/gui/phptmp/"
        </Directory>
    </IfModule>

</VirtualHost>

3.) Restart Apache

# /etc/init.d/apache2 force-reload

Now _every_ customer can login to the webmail through "webmail.customer-domain.tld"

If you use PHP5 you have to change this of course... - take a look in the original <virtualhost>-Part of ispcp (above...)

Thanks @ Joximu!