Thankyou very much for your detailed reply. I really appreciate it. I now fully understand the flow and the connectivity between the RIM's network and Black berry user.
Now I have a last qeneric question i.e for people who have a blackberry device and have exchange in their company but do not have provision to install BES inhouse so does any company provide this service i.e. if the users receive any email in their inbox it can be received in their blackberry as well and they can reply it too - Just wanted to know if there is any hosting service available for this scenario ..
Thanks once again for all your help and excellent guidance.