Many of u ar tryng to change the domain.tld/webmail to webmail.domain.tld.
So I have tryed for adout a week to come up fith something so...here ar the results.
After installing squirrlemail there is a comand line in the how to install ISPConfi 3 tutorial and namely this
ln -s /usr/share/squirrelmail/ /var/www/webmail
. ok u can make that comand and configure you mail client. After that make a new client in ISPConfig with a new site and the domain webmail.domain.tld.
After this the folder /var/clients/client(nr)/web(nr)/web is creted "where nr is number".
ln -s /usr/share/squirrelmail/* /var/clients/client(nr)/web(nr)/web
Now seting permisions
chmod +x /var/lib/squirrelmail/*
chown -R web(nr) /var/lib/squirrelmail
So thats about it.
Hope it works for u to.
If this works send some feedback or thanx