jawad.
there are a 2 options, first you need to decide if you want forward or reflecting mode.
next you need to get between the exchange server and the outbound egress. also you need to look at how many messges per sec you are pushing as multiple NP for email may be required. i have used a internal netscaler to round robin to load balance them previously and mx records with a cost count from 10 to 40.
i would review all this with your client post here is you need more info