It's very usual in control panels to give to the customers an url like webmail.domain.tld (domain.tld is the customer domain)

In ispCP you are usually going to:
http://admin.host-domain.tld/tools/webmail/

With the following changes, you will be able to connect to your webmail through webmail.domain.tld

You have to make changes to two files:
====== Change the dns template ======
The dns template is located at /etc/ispcp/bind/parts/db_e.tpl
In this file you have to add the lines:
<cli>
; CNAME for webmail
webmail                IN              CNAME           www
</cli>

====== Change de 00_master.conf ======
You have to create a new virtualhost in the default site (00_master.conf)
This virtualhost will be the one to handle webmail.domain.tld
You only have to add this after the </VirtualHost> tag

<cli>
<VirtualHost {BASE_SERVER_IP}:80>

    ServerAdmin     {DEFAULT_ADMIN_ADDRESS}
    DocumentRoot    /var/www/ispcp/gui/tools/webmail

    ServerName      {BASE_SERVER_VHOST}
    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/
        ScriptAlias /php5/ /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>
</cli>

====== The expert mode ======
Another way to do the same instead of editing files by hand is to use the attached diffs in bug number #1364
http://www.isp-control.net/ispcp/ticket/1364


====== Related information ======
  * There is a new article created at the forum so you can post your problems
http://www.isp-control.net/forum/accessing-webmail-through-webmaildomaintld-t-3619.html
  * An old post about the topic was on http://www.isp-control.net/forum/trac-spam-t-342-3.html
  * There was an article on the old wiki at http://www.isp-control.net/ispcp/wiki/customer_webtools