Email for any domain user can be checked by going to the domain name url followed by /webmail. 

Example, http://www.yourdomain.com/webmail.  This will redirect the user to a login page for webmail access.  Enter your main email address as the login id. 

If you download email to a PC mail client like Outlook or Outlook Express, you will only see new email that has come into the account that has not yet been downloaded to your PC mail client.  You will not see all the email you have.  By downloading mail to a mail client you free up your email quota space and save room.