Authorize.net does not allow query strings for silent post URL?
The culprit was the iThemes Security plugin. Here are the settings you should disable to avoid getting a 403 forbidden error that prevented the IPN from registering.

Under Banned Users


Hack Repair Default Blacklist


Under System Tweaks


Suspicious Query Strings
Long URLs


I also enabled Enable XML-RPC under Wordpress Tweaks. I had encountered failed IPN's from PayPal before so I assume this would be wise to enable as well.