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