The link in the cPanel for webmail will log you into webmail using the default account. You will in fact be able to see all email accounts attached to your Hosting account by;

Accessing Horde,
Clicking on the plus sign next to the INBOX to display the sub folders,
Clicking on the email account name that you wish to view.

This will then display the Inbox for that email account.

Disadvantages

If you have more than one person with email accounts, you probably wont want them to access and view all the other email accounts on your hosted account. The best way to avoid this is to log in to Webmail directly, by going to yourdomain.com/webmail/.

See the entry in the email section "How do I access WebMail for my account?"