Bounce Rate
The share of sent emails that fail to deliver. High bounce rates signal a dirty list and directly damage sender reputation and inbox placement.
Bounce rate is the percentage of emails that come back undelivered. Hard bounces are permanent failures — the address does not exist — while soft bounces are temporary, such as a full mailbox. A rising hard-bounce rate is the clearest sign a list was not verified, and it is the metric mailbox providers watch most closely.
Keeping bounce rate low protects deliverability for the whole sending domain, because high bounces tell providers the sender is mailing stale or purchased lists. Verifying addresses before sending, removing hard bounces immediately, and avoiding old lists keep the rate in the safe low single digits where inbox placement stays healthy.
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