Ajax Post to guestEntries/saveEntries from another domain
TL;DR: Turns out this was an issue with my apache config on the Craft instance.

Explanation: I did a little digging around the GuestEntriesController.php file and noticed that Craft was not picking up that my POST was coming from an ajax request. After looking into that more I manually added the ajax header to my post request: 

$.ajax({
    url: config.default.apiUrl,
    type: 'POST',
    data: data,
    headers: {'X-Requested-With': 'XMLHttpRequest'}
}).done(function(){ ... });


This got me a little closer, but now on request I was getting hit with this error:


  Request header field X-Requested-With is not allowed by Access-Control-Allow-Headers in preflight response.


That led me to a simple Google search that had the answer in another Stack Overflow question.

In my Craft App's .htaccess file I added in the following 2 headers:

Header set Access-Control-Allow-Origin "*"
Header set Access-Control-Allow-Headers "X-Requested-With, Content-Type"


Now my POST request from another server fires fine, and Craft returns with a successful json response.