Correct me if I ma wrong........
Unna,
if u r using
www.vsnl.com then they are having their own mail host. You need to use that smtp host.
Now when you post
anything@yahoo.com,
it will connect to the smtp host you specified.
That smtp host will connect to the gateway of the vsnl,through the routers.
Now ur request is at the gateway, this will distribute ur request depending on the address u specified, ie yahoo.
That means at the time of coading ur mail client u need to specify only ur smtp host.
Shashank