First of all you need to identify the Salesforce ID for the respective MassMailer Email Statistics Object or the 'Campaign' object in Salesforce by navigating to the record and copying the Salesforce ID from the Browser URL as shown in the following picture.
Following picture shows how to capture Salesforce ID for a MassMailer Email Statistics object.
Following picture shows how to capture Salesforce ID for a campaign record.
Now log into your SendGrid account and navigate to: Stats -> Category Stats
Enter the Salesforce ID in the search box that you got from the previous step and select the result to display the category statistics.
Following images show the steps to do the same. You can export the results from this page if needed.