When you access an Exchange Server from your domain account, you must be authenticated.
If the Exchange Server is a BDC for the users domain, no additional traffic is generated. If the Exchange Server is not a BDC or the client is from a trusted domain, pass-through authentication must find a domain controller. If you have WINS, a broadcast of up to 25 domain controllers will sent. The first computer to respond will be the authenticating server, even if it is remote or not on the same network segment. This process is responsible for all that network traffic.
The solution is to place a BDC on the same network segment as the Exchange Server and to use tip 463 to insure local authentication.