The bounce rate is the share of sent emails that couldn't be delivered. A hard bounce is permanent: the address doesn't exist or the domain has no mail server. A soft bounce is temporary: a full mailbox or a server that's busy.
Why it matters for cold email
Inbox providers read a high bounce rate as a sign that a sender doesn't know who they're emailing, which is how spammers behave. A few campaigns with many bounces can damage a new domain's reputation quickly.
A healthy bounce rate
Keep it under 2%. Above that, stop sending and look at the list: an old source, guessed addresses or a list nobody verified are the usual causes.
How to keep it low
- Verify every list right before sending it.
- Remove hard-bounced addresses for good.
- Be careful with catch-all addresses, which can't be confirmed.
How to work it out
Divide the emails that bounced by the emails sent. Six bounces from 300 emails is 2%, which is the line. Count hard bounces on their own as well: they're the ones that come from a bad list.
Checking a list before you send
Run a single address through the email validator, or a whole list through an email verifier, which asks each mail server whether the mailbox exists without emailing anyone.