By default, all accounts are setup to display the contents of the file missing.html in your main "www" directory when a visitor tries to go to a page that cannot be found (known as a 404 missing error). You may customize this file to display your own custom missing page.
Note that Internet Explorer has a requirement that the 404 missing page be at least 512 bytes in size in order for it to be displayed. This works out to about 10 full lines of text and html code.
If you would like to use a custom page for 404 errors, let us know the new filename and we can adjust this setting.