Your users do not have to go through CPanel. If your domain name has propagated, use http://domain.com/webmail/. Prior to that, you can use http://your-ip-address/webmail/. At the prompt box which comes up, the user must enter the following:
User Name: the full email address (email@example.com)
Password: the password for the email account in question
You can restrict which email program your webmail users are allowed to use by doing the following:
(a) Login to CPanel
(b) Click on "Subdomains"
(c) You will see a blank form to fill out with one next to it saying ".domainname.com". In that blank spot enter what you want to use as your subdomain. For example, "webmail". It may take a minute. So be patient!
(d) Once you get a confirmation message, click the "go back" link (do not use the back button!).
(e) In the line below the one you just filled out, select the webmail.yourdomain.com in the box and click on "Setup Redirection".
(f) In the page that loads next, enter one of the following URLs for the program you want users to be able to access. The trailing slash must be used.
(g) Horde: http://www.domain.com:2095/horde/index.php/
(h) Squirrel Mail: http://www.domain.com:2095/3rdparty/ squirrelmail/index.php/
(i) Click on "Save".
(j) Try and access webmail.yourdomain.com Login with your username and password, you should automatically be at the program of your choice!