Bounce rate

What the number is made of

Bounce rate is bounces divided by emails sent, over a window. It blends two very different things, so the headline figure hides the part you can act on:

• Hard bounces are permanent: the mailbox does not exist or the domain does not accept mail. These come from bad data and are the ones that damage reputation.
• Soft bounces are temporary: a full mailbox, a server that is down, greylisting. These usually resolve themselves.

Track the two separately. A 3% rate that is all soft bounces is a quiet week; 3% hard bounces is a list problem.

Where the thresholds come from

There is no official number, but the working convention in cold email is to keep hard bounces under about 2% and to pause and investigate if a campaign goes meaningfully past it. Mailbox providers do publish limits for spam complaints, not bounces: Google's sender guidelines say to keep the reported spam rate below 0.10% and never reach 0.30%. See complaint rate.

How to lower it

1. Verify addresses before sending, not after. The email verifier checks the mailbox, not just the domain.
2. Suppress every address that hard-bounced, permanently.
3. Re-verify older lists before reuse: data decay turns a clean list into a dirty one on its own.
4. Do not ramp volume faster than a new domain can earn trust. See warming up a sending domain.

Put your own numbers into the bounce rate calculator, and read what a high bounce rate costs.