The term "engagement" that is seen in mailing reports refers to a formula involving mailing recipient actions (opens, clicks, forwards, unsubscribes, etc.) divided by the number of recipients of that mailing. This score is commonly used to gauge recipient interaction for the mailing as a whole and to identify mailings that may not be as effective.
1. It is important to note that the engagement score is an index or summary of combined actions, and not measured on a set scale (i.e. on a scale of 1-10).
2. It is also important to understand how audience and mailing type can return a very different measurement of engagement. Compare "apples to apples" when using the mailing engagement score to determine conclusions of your mailing success.