You can use Get-Message Tracking Logs and Log Parser to calculate and export daily email traffic reports in your infra. Please walk through this article that provides step-wise procedure : http://exchangeserverpro.com/daily-email-traffic-message-tracking-log-parser/
To calculate hourly email traffic reports, you can follow this resource :
http://exchangeserverpro.com/calculate-hourly-email-traffic-using-message-tracking-log-parser/
Moreover, if you want to get this reports automatically into real time, you may consider on
Lepide exchange reporter tool that would be an appropriate approach for you. It will let you to collect email traffic reports of the entire network with the detail information such as, all emails sent and
received within a particular duration, emails sent outside and inside organization, interaction between users in a particular duration etc.