Get a Spam Metric for current and past month

For our quaterly and annual reports, we need to meassure the quantity of Spam tickets we received. Right now we are manually counting the Spam tickets, month by month, in order to have this metric at the end of each quarter. Unfourtunately there is no way to track this for all previous months and we have no way to complete our annual report.
A Spam Metrics is usefull for us to meassure the percentage of real tickets over the total amount of emails received and allow us to make different estimations along time.
Having this metric available will not only avoid the manual counting we are making but also will help us to acces to previous months information.