Recipient grading is a scale to determine the quality of each recipient inside your list. The grade itself is based off of several metrics we track while you send campaigns and users interact with your messages. Recipeint grading uses the following metrics to calculate the grade: recipient source, open average, click average, and recentness of bounces, opens, and clicks. The scale runs from A, B, C, and D. A being the highest of the grades and D being the lowest of the grades.
To see the grade applied to a recipient simply navigate to your Lists tab, click on the name of the List the recipient resides in, and then click on the recipient's email address. In the details section for that recipient you will find a grade applied to that individual address. Keep in mind that recipient grades are a moving scale. For example, an A grade recipient can regress into a C grade recipient over time.