Bounce rate is the percentage of emails you send that a receiving server refuses to deliver. It is calculated as total bounces divided by total emails sent, multiplied by 100. A bounce means the message never reached the inbox: the server rejected it instead of accepting it. Bounce rate is one of the clearest signals mailbox providers use to judge whether a sender is managing their list, which is why keeping it low is central to email deliverability.
Hard bounce vs soft bounce
Bounces come in two kinds, and they mean different things.
- Hard bounce. A permanent failure. The mailbox does not exist, the domain is invalid, or the server is blocking you outright. Servers do not retry a hard bounce, and these are the bounces that damage sender reputation fastest. Almost all of them trace back to a bad address.
- Soft bounce. A temporary failure. The inbox is full, the message is too large, the receiving server is briefly overloaded, or a new sender is being greylisted. Servers will usually retry, and many soft bounces clear on their own. An address that soft bounces repeatedly, though, should be treated as undeliverable.
The split tells you where to act. Hard bounces are a list-quality problem you fix by verifying addresses before sending. Soft bounces are a sending-behaviour problem you fix with warming, suppression of repeat offenders, and patience on retries.
What is a healthy bounce rate?
A healthy total bounce rate sits under 2 percent. Useful bands:
- Under 1 percent: excellent.
- 1 to 2 percent: acceptable.
- 2 to 5 percent: concerning, worth investigating your data.
- Over 5 percent: dangerous. Providers begin routing your mail to spam, including mail to valid recipients.
By type, healthy hard bounces stay under roughly 0.3 to 0.5 percent and soft bounces under about 1 to 1.5 percent. On cold outbound the bar is tighter, with most teams targeting under 3 percent total and treating anything higher as a sign of bad source data. The damage is not linear: a list bouncing around 6 percent can drag open rates into single digits within weeks, and reputation recovers slowly once it is hit.
How verification lowers bounce rate
Most hard bounces come from addresses that were already invalid when the list was built. Verification opens an SMTP conversation to check whether each domain and mailbox exists, without sending a message, so you can delete the undeliverable rows before they ever bounce. Removing those would-be hard bounces is the single most direct way to pull a bounce rate into the safe band and keep it there. It is the cheapest lever on deliverability and the first one to pull.