Hi the form at bottom of the page points to the /subscribe/ page, this is not the Newsletter php page that collects subscription. The subscribe.php is giving a 404 probably because file systems permission that don’t allow the web server to activate that page. More, the web server is badly configured since it allow file listing:
http://www.macknewton.com/wp-content/plugins/newsletter/
Try to check the file permission in your wp-content folder.
Bye, Stefano.
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