What is bounce rate? (Email glossary definition)

Bounce rate is the percentage of email messages that fail to reach the recipient's inbox, expressed as bounced divided by total sent. The denominator is the number of messages successfully accepted by your ESP for sending. The numerator is the number that failed at any point during delivery.

Bounce rate is the single most important deliverability metric most marketers underwatch. ISPs treat it as a direct signal of list quality. High bounce rate triggers sender-reputation demotion, which routes your future mail (even to good addresses) into spam.

Two kinds of bounces:

• Hard bounce. Permanent failure (5xx code from the receiving server). Address does not exist. Remove immediately.
• Soft bounce. Temporary failure (4xx code). Mailbox full, server unavailable, temporary block. Retry, then suppress after 3 consecutive failures.

Healthy bounce rate is below 2%. Above 5% is actively damaging. See what is a good bounce rate for the full thresholds.