CRM IFD Authentication Failed Discovery.svc

I have the following set up on CRM 2013

internal DNS is set up to resolve the external domain internally (forward lookup zone) and allows access via crm client internally and web internally.

Externally I can access CRM without issue through the web browser over https, however when I try to configure the crm outlook client for external users (i.e. non domain computers) I get a failure.  Inspecting the log file I can see an authentication failure on https://crm.<domain.com>/XRMServices/2011/discovery.svc

I tried allowing unauthenticated access to the XRMServices folder through IIS but then discovered an Authentication Required error in the log file.

I assume the issue is with accessing the components with a non-domain user account initially as computers that are members of the domain work fine.

Is there any way to resolve this issue?