MS Exchange in LTM Arm Mode

The easiest way to do this is to set the default gateway of the exchange server to the floating ip of the LTM on the same subnet then make sure you don't have SNAT enabled on the virtual server.

If you used a SNAT you would need to use the x-forwarded-for header in the HTTP profile and install the ISAPI filter on the servers but that only logs the true client IP not present it to the application.