Message metrics are now based on the number of delivered messages
In the past, we based all of our metrics on total messages sent. By default, we now base metrics for message statuses that occur after a message is sent (e.g. opened, clicked, etc) on the Delivered metric—the number of messages that actually reach your audience’s inboxes. This helps provide you with a more accurate understanding of how your messages perform.
However, if you don’t pass delivered metrics from your delivery provider to to Customer.io, you can continue basing metrics on the number of messages sent. You can change your Metric Configuration on the Custom SMTP settings page.