Secure Mail for WindowsPhone does not work when entering Credentials the App states "Access to your company network is not currently available" Android and iOS devices are working fine and are able to connect to Secure Mail.
Direct native mail clients are accessible.

Changing the backend server certificate to use 2048 bits key size was able to fix the issue.

Problem Cause

The Secure Mail for windows does not connect to exchange server. This is because the traffic goes through the micro VPN and there is connectivity issues with the SNIP to the exchanges server. 

The Android and iOS devices connect fine with WM as the Network Access is “Unrestricted”. When they are connected via “Tunnelled to internal network”, they face the same issue as such.

On diagnosing found that the SSL cert used for exchange is of 4096 bits which is not supported with NS VPX for backend communication. 

While changing the backend server certificate key size to 2048 bits was able to enroll the devices.