[imp] How to restrict login to IMP to a list of users ?

[Bianchi Quota Leonardo]

I have a Mail Server (RH7.3) for 200 users and they all use outlook as mail
client. Only 20 priviliged users are going to use IMP (as extra email
service) and for this reason I would like to go through the implementation
of mechanism of authorization only for them. Infact unfortunately all 200
users have a problem of weak password. To solve partially this problem I
would like to:
1)change the password for these 20 persons. (reconfiguring Outlook)
2)restrict the access to IMP only to these 20 person

I don't know how to realize this second 2) task. 

Both a suggestion or instructions step by step are welcome. What I have to
change in IMP or Horde config files and more... ?

I have installed also these rpms:

imp-3.2.1-1.noarch.rpm
horde-2.2.3-1.noarch.rpm
mysql-3.23.41
imap-2000c-15

At this moment I configured IMP with https (and not http) disabled IMAP
functionality and enabled pop3s.

Thanks.

Leonardo.