find the size of transported emails in an exchange organization in a period of time

Hi

it may seems weird but anyway i need to know the size of traffic flow in my exchange organization in a specific period of time.

for example i like to know how much (in GB or MB) mail flow has passed using our transport servers in the last three months and it will be very good i f i can find how much of that has been sent to edge and how much has been handled inside the organization and our accepted domains

thanks