There is an initial communication from the client (on an ephemeral port) to the server's TCP port 135. The server's RPC endpoint mapper establishes a random port on the server and sends that to the client and then the client connects to that random port.
Here's an article from Microsoft that talks about what you'd need to do to establish static ports on the server: http://support.microsoft.com/kb/270836
If you want to quickly see if the server has this configured, go have a look at
HKEY_LOCAL_MACHINE\System\CurrentContolSet\Services\MSExchangeIS\ParametersSystem and look for a value called "TCP/IP Port". (That's only one of the places where it would need to be set. The article can tell you the rest.)
(If you're looking for this to forward Outlook-to-Exchange communication across a firewall for a large number of clients think about using RPC-over-HTTP(S) (also known as "Outlook Anywhere") in lieu of making this change. If not, forget that I said anything...)