Cpanel Webmail Access
To access webmail with Cpanel,
1) Select 'Webmail' > 'Go to Secure Webmail Login'
2) Select an email option; such as 'Horde'.
3) Select an email address from the list. At this point, using one of the email services is much like using a service such as hotmail or gmail.
As far a your clients are concered, they may want to access their webmail with their domain. It looks a little cleaner. One such trick is to add their domain into the url. For example, http://example.com:2095/ could be used to access webmail for any account at example.com; cuch as firstname.lastname@example.org.