You can access your email directly by going to yourdomainname.com/webmail
Your user name is email@example.com. Your password is the password assigned to your mailbox in the mail manager. Remember that passwords are case sensisitive.
Webmail access is also available from the cpanel. Click on the Email Accounts button, then click on the More button in the right hand column corresponding to the email address you wish to access and choose Access Webmail.