Bounce Rate
Email bounce rate measures reported delivery failures among attempted messages. Separate final failures from temporary deferrals and spam-folder delivery.
Email bounce rate is the percentage of attempted recipient messages reported as delivery failures. A message delivered to a spam folder has not bounced merely because it missed the inbox.
Count failures consistently
Divide bounced recipient messages by attempted recipient messages and multiply by 100. Use the same sending cohort and reporting window. A provider may receive a failure notice after its initial report, so a recent send's rate can still change.
Check how your provider treats temporary deferrals. A queued retry is not necessarily a final failure, and counting every retry as a new email can distort the denominator.
Handle the reason for the bounce
A hard bounce reports a permanent failure, such as an invalid address. Stop retrying a known invalid address. A soft bounce reports a temporary problem. Follow the provider's retry policy and inspect the response if it persists.
Review failures by recipient domain and response code. A cluster at one provider calls for a different investigation from invalid addresses throughout a new import. Authentication failures and policy rejections also need diagnosis. Deleting addresses will not repair the sending configuration.
There is no single bounce-rate threshold that establishes good deliverability. Compare similar sends and investigate new failures even when the overall percentage looks small.