squirrelmail and ispconfig

[munika]

i have FC5 and have setup ispconfig. everything works fine. so i have http setup and i have ftp and mailserver setup with the help of "Ther perfect setup FC5"

i now want to install a webmail for my domain name www.keshwani.net
i got this howto from the howtoforge.com website.
http://www.howtoforge.com/squirrelmail_ispconfig

I have a question. The how-to says to setup a site webmail.keshwani.net and then create a user and then install the software. it is fine.
On www.keshwani.net i have some emails/users already setup which are using thunderbird (email client) to send and receive. will these users be able to log in to webmail.keshwani.net if setup according to the how-to and send and receive emails from both thunderbird and from webmail.

how will the users under webmail.keshwani.net and www.keshwani.net be one(shared). do i have to replicate users. if users replicated how will they get emails. will it to go webmail inbox or will it go to the postfix inbox.

please do let me know
thankyou

munika