CentOS4 MailScanner virtual users and webmail solutions

[Dave]

   Hello,
First of all thank you greatly for the rpm of MailScanner it installed just 
fine on a CentOS4.x server.
We've got an old sendmail box running on fc2 that we're going to be
upgrading to postfix running on CentOS4. We host literally hundreds of
domains, some with a single user others with multiple users. Currently our
sendmail setup has each user with a Unix account, needless to say our
password files are huge. We don't have a lot of overlap between usernames,
i.e. we don't have two user1 users so currently that isn't an issue. I'm
wondering given our setup, which would be better virtual alias or virtual
mailbox domains? I'm leaning toward virtual mailbox, but would appreciate
some practical experiences. In addition, a majority of our user's either get
their mail via pop, i think now is the good time to add authenticated smtp
with tls for this, and others prefer a webmail solution. We use usermin, (I
did not set that up) and i want to move away from this. I'm thinking
squirrelmail, but am not sure if it'll work with our virtual mailbox 
domains.
Finally we'll be adding a web control panel so user's can administer more of
their own accounts, plesk i was last told.
    I'm looking for any compatibility issues i might encounter with all
this, any practical user experiences with this or similar setups, and any
alternative webmail or setups to consider. I am particularly nervous about
moving the customers away from sendmail, as i don't know it at all and not
sure how it's set up, though i'll be relieved when it is done.
We're also doing MailScanner and antispam and antivirus, i'd prefer to stick 
with rpms as much as possible to keep consistency with packages.
Thanks a lot.
    Dave.