XML : Manual autodiscover implementation failing. Why?

I am managing a mail server, and would like to have my Outlook users be able to automatically set up their mail server information. Normally this is done with an autodiscover file. Here's how I understand it works with mymaildomain.com as an example (not the actual domain):

• Host a file at mymaildomain.com/autodiscover/autodiscover.xml (https) with the correct information
• User enters foobar@mymaildomain.com and their correct password
• Outlook then tries to fetch the autodiscover XML from one of the following URLs:
  • https://mymaildomain.com/autodiscover/autodiscover.xml
  • https://autodiscover.mymaildomain.com/autodiscover/autodiscover.xml
• Outlook uses the information in that file to populate the correct fields like IMAP, POP, and SMTP server names, ports, etc.

I've got the file hosted (pasted below) and when I open a browser or use curl to GET or POST the URL, I get HTTP/200 and the XML is returned properly. When I try it in Outlook for Mac 15.17.1, it doesn't work. When I use the Remote Connectivity Analyzer, it gives me the following error:

  The Microsoft Connectivity Analyzer is attempting to retrieve an XML Autodiscover response from URL   https://mymaildomain.com:443/Autodiscover/Autodiscover.xml for user   testlogin@mymaildomain.com.        The Microsoft Connectivity Analyzer failed to obtain an Autodiscover XML response.    Additional Details    An HTTP 500 response was returned from Unknown.  HTTP Response Headers:  Connection: close  Accept-Ranges: bytes  Content-Length: 2524  Content-Type: text/html  Date: Thu, 17 Dec 2015 22:15:53 GMT  Server: ""  Elapsed Time: 582 ms.    

What's baffling is that any other method I use to request the autodiscover.xml file, I'm able to get it. So why can't Outlook or the Remote Connectivity Analyzer fetch it properly? XML file below.

Thanks

  <Autodiscover xmlns="http://schemas.microsoft.com/exchange/autodiscover/responseschema/2006">  <Response xmlns="http://schemas.microsoft.com/exchange/autodiscover/outlook/responseschema/2006a">  <Account>  <AccountType>email</AccountType>  <Action>settings</Action>  <Protocol>  <Type>IMAP</Type>  <Server>imap.mymaildomain.com</Server>  <Port>993</Port>  <DomainRequired>on</DomainRequired>  <SPA>off</SPA>  <SSL>on</SSL>  <AuthRequired>on</AuthRequired>  </Protocol>  <Protocol>  <Type>POP3</Type>  <Server>pop.mymaildomain.com</Server>  <Port>995</Port>  <DomainRequired>on</DomainRequired>  <SPA>off</SPA>  <SSL>on</SSL>  <AuthRequired>on</AuthRequired>  </Protocol>  <Protocol>  <Type>SMTP</Type>  <Server>smtp.mymaildomain.com</Server>  <Port>465</Port>  <DomainRequired>on</DomainRequired>  <SPA>off</SPA>  <SSL>on</SSL>  <AuthRequired>on</AuthRequired>  <UsePOPAuth>on</UsePOPAuth>  <SMTPLast>on</SMTPLast>  </Protocol>  </Account>  </Response>  </Autodiscover>