When checking "Disable Public section" under Settings, it's important to know what parts of the public section are actually disabled.
Assuming the domain for your email marketing software is: my-site.com/em/, users that visit that URL with "Disable Public section" checked will be redirected to the admin section: my-site.com/em/admin/.
If, however, a user clicks on a link in a mailing, or is otherwise directed to a specific public page, such as my-site.com/em/archive/list-a/43 (public message archive page for a specific message), the page will still load, but the main site navigation will not appear.
This is necessary because although the public section is disabled, the interface still needs to show users specific pages for things like "Forward to Friend," "Web Copy," which can appear in mailings. Although the user can view these specific pages, they cannot navigate elsewhere since the main navigation links do not appear.
