Help on Last Setup

[mcristian@libero]

Goordmorning Horde & IMP Team,
             i tryed to install horde and imp from source and i had 
some trouble, so i use the rpm packages from horde.org and i have no 
problems, but now i would like to have some infos: I would like to 
create a webmail server and i have all i need but i didn't understand 
how to configure horde and imp to work with my sendmail.
I have apache 1.3.14, sendmail 8.9.x, imap (from your site) correctly 
running and working. I follow all the instructions to configure horde 
and imp but now when i'm doing the setup.php3 i can't understand how to 
setup the mailbox for my existent users and the kind of database i 
should use (if i really need it) for this webmail.
Can you help me ?

Thanks in advance
Cristian Marchionni