You can find out the expected client network traffic between Outlook 2007 and Exchange Server 2007/2010 using the calculation method below.
First, determine the level of activity for a user, using the following table.
For example, a user with medium activity will send 10 messages per day, receive 40 messages per day, read every message received and delete half of the messages. Logon and logoff costs from client types other than OWA were not evaluated, because enterprise email users generally stayed logged on for days at a time.
Now you can evaluate bandwidth usage of a specific user. In the following table, values are in kilobytes per day per user. The sending portion has been separated from the other actions, which are labeled "aggregate".
For more detailed information about calculation of bandwidth usage with Exchange, refer to the following Microsoft article: http://technet.microsoft.com/en-us/library/cc540453(EXCHG.80).aspx.