Hi - I got the reason why POST is not working.. Seems like POST doesn't work when your target JSP redirects to another URL.
You should use client-side fetching as a fall-back option when an iView is unable to display content using the server-side fetch mode. These problems are often associated with encoding and scripting issues at the source or in the URL. For example, the server-side fetch mode does not support URLs that are automatically redirected to another URL
Reference : https://help.sap.com/saphelp_nw04/helpdata/en/8d/8e0c7e477e4239a7ee288d7846a0bb/content.htm
Mapped user and Mapped password does work.
I've created an abstract portal interface in which i'm reading the current user ID and making a call to the external JSP and fortunately it's working..