To properly use jInbound, you must create a menu item for the extension in the backend Menu Manager.
Attempting to viewing a landing page before you've created the jInbound Menu Item may result in a 500 error.
However, many users do not want a public menu item for your landing pages as you are likley promoting the landing page URL directly with print media or via social media and calls to action.
Thus, a common solution is to create a Hidden menu in the Menu Manager. Then, create a jInbound Menu Item in the Hidden menu.