This refers to the rate or number of emails sent without bouncing. It is estimated by dividing the number of sent by the number of those delivered.
Calculating an Email Delivery Rate
When you divide the number of emails sent minus bounces by the number of sent emails, what you get is the email delivery rate. Note that the number of sent emails is different from the number of emails delivered, considering some of the emails may have bounced, and are not counted as part of the emails that are delivered to the recipients’ inboxes.