Click the Email Accounts icon
To access your webmail for the main account, you would use this link, but we're not going to do that
Instead, click More
Then click Access Webmail
Type the password for this email address NOT your cPanel password
Now click Log In
Click the webmail application available to you... in this case we'll select Horde
This is the Horde webmail main screen, where we can check our inbox...
... and compose a New Message
We're not going to get into the specifics of using the Horde webmail application... but suffice it to say that this is how you access, and manage your emails from within cPanel
When finished with web mail, click Logout