Bounce Rate
The percentage of sent emails that couldn't be delivered to the recipient's inbox.
Bounce rate is one of the most critical metrics in cold email. A high bounce rate signals to email providers that you're sending to invalid addresses, which damages your sender reputation.
There are two types: hard bounces (permanent failures, like invalid addresses) and soft bounces (temporary issues, like full mailboxes). Hard bounces are the dangerous ones — keep them under 2% of total sends.
To maintain a low bounce rate: always verify your email list before sending using tools like ZeroBounce or NeverBounce, remove bounced addresses immediately, and avoid purchased email lists.