Bounce rate
The percentage of sent emails that couldn't be delivered to the recipient inbox.
Bounce rate is the percentage of emails in a send that failed to be delivered to the recipient's mail server. It's calculated as (bounces / sends) × 100. Healthy email programs maintain bounce rates under 2%; anything above 5% triggers deliverability warnings from ESPs and ISP throttling.
Bounces split into two categories: hard bounces and soft bounces. Hard bounces are permanent failures — the address doesn't exist, the domain doesn't exist, or the recipient server has explicitly blocked you. Hard bounces should be removed from your list immediately. Soft bounces are temporary failures — the mailbox is full, the server is temporarily down, the message was too large. Soft bounces can be retried.
High bounce rates almost always indicate a list-hygiene problem. The two most common causes: list aging (subscriber emails become invalid as people change jobs and email providers retire addresses) and list acquisition issues (purchased lists, scraped lists, or sign-up forms without confirmation steps). Regular list validation and double-opt-in are the structural fixes.