Authenticate using "Transparent" mode behaviour

Hi All,

We have the requirement to consume the webmail of our organization through API gateway.It uses the NTLM authentication.
The below configurations has been made in the API gateway,
The target endpoint is the REST url
The policy “Outbound Authentication Transport” has been used with Authentication Scheme as NTLM and the Authentication Mode as Transparent.

As quoted in documentation ,The NTLM simply passes the incoming credentials to native API.This works fine when I try invoking the API from SOAP UI.

But when I try invoking it from the webMethods IS (another instance) am getting 401-Unauthorised.

NOTE:When I try invoking the endpoint url (webmail url) directly in IS it works fine.[i][u]